Abstract: The discrete step-type phase modulation and pixel gaps of liquid crystal spatial light modulator (LC-SLM) are the main distortion reasons for the 4f pulse shaping system. Based on the analysis of the above factors, the 4f pulse shaping system mathematical model is established and simulated with different modulation rates, different pixel sizes, different gap sizes and different dispersion relations. The simulation results show that step-type phase modulation can lead to replica pulses, the modulation rate or pixel size is larger, the intensity of the replica pulses is stronger, and the distortion of shaped pulse is more serious; the pixel gaps can also lead to replica pulses, the gap size is larger, the intensity of the replica pulses is stronger, and the distortion of shaped pulse is more serious. The interval of the replica pulses is a constant determined by the frequency separation of adjacent pixels. The nonlinear spectral dispersion can reduce the intensity while causing chirp effect of replica pulses.