A two-stage high power narrow linewidth fiber amplifier was demonstrated. The first stage was single mode Er3+ doped fiber amplifier. The double clad Er3+/Yb3+ co-doped fiber amplifier was designed as power amplifer in which two 980 nm high power laser diodes were used as the pump sources.With the maximum pump power of 10.7 W, the amplifer output powers was 1.94 W and the optical-optical conversion efficiency was 20%, the slope efficiency was 17%. The 3 dB linewidth of seed laser and each amplifier were measured by the delayed self-heterodyne method with 10 km single mode fiber. The results show that obvious spectral broadening happenes during the amplification process. The reason of spectral broadening is owing to self-phase modulation caused by intensity fluctuations of relaxation oscillations or self-pulsing.