The stimulated Brillouin scattered (SBS) light caused by laser irradiation at target is modulated when passing through a continuous phase plate (CPP), which causes damage to optical elements. In this paper, a theoretical model for analyzing back near-field diffraction transmission of SBS light is established, which is combined with Fresnel diffraction theory and used to analyze and numerically simulate the influences of diffraction distance, CPP parameters and application position on near-field diffraction modulation. The results show that as the diffraction distance increases within a certain range, the near-field modulation increases rapidly. Near-field modulation is affected by CPP phase amplitude, minimum spatial period and spectral control, in which phase amplitude has a relatively high impact. In addition, the effect of doubling frequency CPP on near-field modulation is weaker than that of fundamental frequency CPP. The study of back near-field diffraction transmission of CPP is helpful for understanding the near-field intensity distribution of a phase optical device and provides some guidance on the arrangement of optical elements of a high-power laser system and the optimization of CPP.