The single hundred-picosecond pulse amplification (the repetition rate is 1 Hz) was studied experimently in the Ytterbium-doped double clad fiber amplifier. The characteristic of time-domain and frequency-domain pulse waveform influenced by the self phase modulation (SPM) effect was presented. In the experiment, the small mode area double clad fiber was adopted with core diameter 6.5 μm as the gain media. As the gain of the amplifier from the small-signal gain to the deeply saturated gain, the evolution of the spectrum broadening, the pulse waveform and intensity distribution because of the SPM effect were observed. Then to overcome the difficulty for measuring the single pulse spectrum, the Bragg fiber grating scanning was introduced to investigate the changing of spectrum broadening. The results indicate that different from the SPM effect in the passive fibers, as the increase of the pump power, due to the competition mechanism of the gain saturation and SPM in the hundred-picosecond pulse amplification, the spectrum exhibits asymmetric double-peak and the pulse intensity is shifting to short wavelength firstly and then to the long wavelength.