Based on the theoretical analysis of least mean square (LMS) algorithm, by using the adaptive equalizer with post feedback, the waveform distortion of hundred-nanosecond laser pulse is restrained, and the gain of Er3+/Yb3+ co-doped double clad fiber amplifier is flattened. A distorted waveform and pulse width narrowing are observed after two-stage cascade amplification. Based on LMS algorithm, the distorted waveform can be fitted by an adaptive equalizer. The waveform of the trigger signal is adjusted through calculation to make the top of pulse laser tend toward flat. And experimental verification is performed.