The conventional techniques for preparing fiber gratings involve ultraviolet exposure and CO2 laser methods. The ultraviolet exposure is advantageous due to its simplicity and ease of alignment. However, it typically requires hydrogen-carrying sensitization treatment on the fiber and the refractive index modulation can be easily erased at high temperatures, limiting its applicability in extreme environments. On the other hand, the CO2 laser method is commonly used for producing long-period fiber gratings but its measurement sensitivity is susceptible to high temperatures. To address the limitations of these traditional methods, femtosecond laser scribing technology has emerged. This technology encompasses femtosecond laser direct writing, femtosecond laser holographic interference and femtosecond laser phase mask methods. Among these, femtosecond laser direct writing offers high efficiency, low pulse energy requirements, and the ability to modulate the center wavelength, grating spacingand grating length based on sensing requirements. Moreover, the experimental setup for femtosecond laser direct writing is simple and does not require a phase mask to control the grating period. Femtosecond laser direct writing fiber gratings exhibit flexible refractive index modulation, excellent high-temperature performance, and high mechanical strength. As a result, they have found widespread applications in sensors, lasers, and other optical devices. This paper provides a brief introduction to the working principle and typical writing methods of femtosecond laser direct writing fiber gratings. It also summarizes the research progress of three direct writing methods of femtosecond laser, both domestically and internationally. The advantages and disadvantages of these three methods are compared and analyzed in terms of preparation efficiency and spectral quality. Additionally, the paper delves into the detailed analysis and discussion of spectral optimization methods, including laser pulse energy, grating length, fiber type, beam shaping, and grating apodization. The ultimate goal is to achieve a high reflection peak, narrow 3dB bandwidth, and low insertion loss.