Velocity bunching modulation is the key theory of SAR(Synthetic Aperture Radar) imaging for shallow ocean under water bottom topography.This paper elaborates velocity bunching modulation from the stand point of Doppler frequency shift firstly. Then theoretical deduction is carried out from the standpoint of moving ocean waves, and the differences and similarities from two standpoints are pointed out. Finally,it is verified by theoretical deduction that radar parameters should satisfy certain stipulation conditions in order to guarantee the accuracy of velocity bunching modulation.This work has provided a reference for the design of SAR ocean remote sensing system.