Abstract: An experimental research on self-phase modulation (SPM) of high power picosecond pulse amplifier was conducted by high-power all-fiber picosecond pulse fiber laser with an MOPA configuration. The seed source is a self-built fiber laser passively mode-locked by semiconductor saturable absorber mirror (SESAM). To avoid nonlinear effect, the repetition rate of the seed pulse should be increased to 328 MHz through a self-made repetition multiplier before amplification. The amplifier employed a three-level amplifying structure and finally received a stable picosecond pulse laser output with a center wavelength of 1 066.5 nm, a 3 dB spectrum width of 2.5 nm, and an average power of 91W. In the experiment, there was research on change of spectrum caused by SPM in the process of amplification. An analysis of the output spectrum of the laser demonstrates that with the increase of power, the influence of the initial chirp and the shape of the pulse on the SPM effect of high power fiber laser becomes deepens. In the mean time, it is also influenced by self-steepening effect.