Ultrafast laser refers to a type of light source with pulse duration in the picosecond or femtosecond range and high peak power. It is generally generated from active or passive mode-locked lasers and is widely used in fields such as material processing, biomedicine, and precision measurement. At present, researchers have demonstrated mode-locked lasers based on gain materials such as solid-state crystals, semiconductors, and active fibers with various types of ultrafast laser output. However, due to the limited optical gain inside the laser cavity or severe nonlinear effects caused by high peak power in the waveguide structure, the power and energy of ultrafast lasers that directly generate through mode-locking operation are generally low, rendering it difficult to meet the practical application needs. Regarding the power or energy scaling of ultrafast lasers, rare earth doped optical fiber is recognized as the preferred choice for constructing high-power ultrafast laser amplifiers due to their unique waveguide structure, which implies a large ratio of surface area to volume, excellent thermal optical performance, and high conversion efficiency (electro-optical efficiency can exceed 30%). In recent years, lots of research teams at home and abroad have made significant progress in improving the power and energy of ultrafast lasers based on the technique of fiber chirped pulse amplification (CPA), whilst with the compromise of a de-chirped pulse width >200 fs, owing to the spectral narrowing after amplification induced by the gain saturation effect. An alternative is to leverage the fiber nonlinearity to counteract the gain narrowing effect, such as the pre-chirp managed and self-similar amplification, which is capable of generating μJ level pulse energy with sub-50 fs pulse duration. However, the former involves fine controlling of the chirping parameters of the laser pulse, rendering the system sensitive to changes in amplifier parameters, while the latter is explicitly restricted by the limited optical gain spectrum in terms of further energy scaling. Recently, researchers have proposed another scheme called gain managed nonlinear (GMN) fiber amplification, in which the laser pulse undergoes self-similar pulse evolution firstly and then further experiences spectrum broadening under the effect of active fiber dynamic gain, achieving pulse energy enhancement with a pulse width within 50 fs. Specifically, researches have shown that the fiber GMN pulse evolution is a novel nonlinear attractor that can tolerate nonlinear phase shift up to 200 π, thus having the potential to achieve higher energy ultrafast pulse laser output. First, the technical principle of the GMN fiber amplifier is introduced through establishing a theoretical model that involves the generalized nonlinear Schrodinger equation, distributed rate equation and power transmission equation. Based on the theoretical model, the evolution of pulse time-frequency characteristics of the ytterbium-doped fiber GMN amplifier are simulated, and the effects of different seed parameters on the laser output are analyzed. According to the simulation, it can be proven that the pulse evolution under GMN amplification mechanism is a new type of nonlinear attractor, which is mainly formed by the nonlinear spectral broadening managed by dynamic gain in the amplifier, and is less affected by the initial pulse characteristics of the seed source.Based on the GMN fiber amplification mechanism, researchers have conducted a series of experimental investigations in recent years and achieved significant research progress. According to theoretical simulation results, GMN pulse amplification generally occurs in positive dispersion optical fibers. Spectrally narrowband ultrafast laser pulses, under the combined effect of fiber nonlinear self-phase modulation and dynamic gain, evolve into laser output with a significant spectral extension to the long wavelength range and can be temporally compressed to the transformation limit with an increasing of the pulse energy. At present, output pulse energy higher than 1.3 μJ with duration in the 50-fs level has been realized from GMN amplifiers that exploit large mode area fiber, while further energy scaling is limited by the stimulated Raman scattering (SRS) effect. Based on the GMN fiber amplification mechanism, researchers further explored its output laser pulse characteristics at ultra-high/low repetition rates from the perspective of practical application requirements. In addition, the spatiotemporal degradation (STD) in GMN fiber amplifiers has also been theoretically investigated, and the results show that the STD has apparent threshold characteristics. When the pump power of the amplifier exceeds a certain threshold, the beam quality of the output laser rapidly deteriorates, Raman noise increases, pulse coherence decreases, and the amplified laser pulse is difficult to compress. The main driving factors are the nonlinear SRS effect and the induced four wave mixing effect between transverse modes. With the overall performance improvement of GMN fiber amplifiers, its application in for example multi-photon microscopy imaging and fine micro-machining has also begun to receive widespread attentions. Moreover, GMN fiber amplifiers have also been used as component of ultrafast laser systems, supporting ultrafast laser output with engineered performance. As a new type of nonlinear fiber amplification mechanism, GMN pulse evolution can realize wider linear spectrum broadening compared with convention self-similar amplification and pre-chip managed amplification, and thus possesses more advantages in laser pulse width narrowing and energy scaling. In recent years, the GMN fiber amplifier has attracted more and more research attentions, and its outputted single pulse energy has reached more than 1 μJ and the de-chirped pulse duration approached 30 fs level, as well as preliminarily realized application in fields such as multi-photon microscopy. Nevertheless, there exist a significant room for the improvement of the output laser pulse performances of GMN fiber amplifiers such as single pulse energy and spatiotemporal properties. To achieve the above goals, it is necessary to carry out more accurate simulation analysis of the pulse evolution process and further clarify the physical mechanism of its high tolerance for nonlinear phase shift. Based on the theoretical research, by using active fibers with larger mode field area associated with optimizing the amplifier structural parameters, it is expected to achieve ultrafast laser output with higher single pulse energy, shorter pulse width, and higher spatiotemporal characteristics, as well as further expand its application breadth and depth.