In order to investigate the characters of the periodic phase-modulating beam amplifying, the model of pulse laser amplifying is improved based on the analysis of the parameters influenced by the transition lineshape of the media, such as the loss, stimulated radiation cross-section and saturated energy-density during the progress of amplifying, and the case of phase-modulated pulse laser in Nd:YLF is studied also. The pulse period of simulation based on the model is 330 ps, which is consistent with the experiment with period of 300 ps. The main factors affecting output time-waveform are studied by simulation, and the effect of phase modulation to amplitude modulation (PM-AM) conversion may be weaken if the center frequency of incident beam matches the center frequency of transition lineshape, which is useful for suppressing the effect of PM-AM conversion.