High-power narrow-linewidth fiber lasers with excellent beam quality have been highly desired for spectral and coherent beam combinations[1-3]. The power scaling of single-mode narrow-linewidth fiber lasers has been under intense investigation[4-15]. This mainly involves comprehensive suppressions of the transverse mode instabilities (TMIs) and nonlinear inelastic scattering effects, such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS)[16-18]. Basic principles to increase the TMI and SBS/SRS thresholds in high-power fiber lasers have already been proposed based on the understanding of those effects[19-20]. Specifically, the TMI effect could be suppressed through reducing the strength of the thermally induced index grating, manipulating the phase shift between the thermally induced refractive index grating and the modal interference pattern, and decreasing the relative gain of the higher-order modes (HOMs)[19]. The SBS/SRS effects could be suppressed through reducing the effective fiber length or the effective Brillouin/Raman gain coefficient[20]. Those principles could be fulfilled mainly through special design of active fiber or optimization of the structure and parameters of fiber amplifiers. As for the fiber design, a near single-mode fiber laser with an output power of 3 kW and spectral linewidth of 12 GHz has been achieved based on gold-coated specialty gain fiber[5], and a single-mode fiber laser with an output power of 3.5 kW and spectral linewidth of 0.18 nm has been achieved based on low-numerical aperture (NA) low-mode-area (LMA) gain fiber[6]. It should be noted that those two fiber laser systems are established in free-space structure, which is not suitable for compact assembling and maintenance. Therefore, most of the reported all-fiberized narrow-linewidth fiber amplifiers are based on conventional active fiber and require comprehensive optimization of structure and parameters of the fiber amplifiers[7-15]. As for all-fiberized format, 3.7-kW output power has been achieved with near-diffraction-limited beam quality and spectral linewidth of approximately 0.3 nm[15].