We report a high power fiber amplifier based on nonlinear chirped-pulse amplification[NCPA].To manage the nonlinearity,pulse shaping is introduced by self-phase modulation in the fiber stretcher with the help of spectral filtering.The third-order dispersion is compensated for by the nonlinear phase shift in the NCPA.With optimization,the system can output 382 fs pulse duration with 20 W average power at 1 MHz repetition rate.The long-term average power fluctuation is measured to be 0.5％ in 24 h,and the beam quality factor[M2]is 1.25.