Amplified spontaneous emission（ASE）is one of the most important factors that reduce the fiber amplifier′s efficiency.In order to restrain ASE,a synchronously pulse-pumped Yb-doped double-clad fiber amplifier is designed,which is proven to have significant effect of reducing the power of ASE.At a repetition of 400 Hz,the power of the signal with pulse duration of 500 ns and average power of 15 mW,is up to 170 mW and a gain of 10.5 dB is obtained.Meanwhile,the impact of different pulse width on the ASE has also been studied.