High-power narrow linewidth fiber lasers exhibit significant potential in precision measurement, LiDAR, spectral analysis, and nonlinear optical applications, thanks to their exceptional reliability, high electro-optical conversion efficiency, good coherence, and high beam quality. In this paper, the narrow linewidth polarized laser is investigated by using seed, multi-stage pre-amplification and main amplification. In the experiment, the narrow linewidth seed light in the 1056 nm wavelength band is amplified in multi-stages with a power output of 20 W, and then injected into the main amplification system with bidirectional pumping. The cladding pump stripping of the laser is directly fabricated on the melting point of the fiber, which can reduce the melting point and simplify the laser structure. By optimizing the ratio of pump power before and after the main amplification system, the output power of laser is 2.505 kW when the pump current is 11.2 A, and the optical-to-optical conversion efficiency of the fiber laser is 83%. And higher power output can be obtained by continuing to increase the pumping power at a later stage.