As one significant aspect of SAR development, simulation of SAR echo signal has an important practical sense in theory and project in aspects of system design and imaging algorithm verification. The component of raw signal simulation system is shown. Then, according to the time domain signal model, the echo of SAR is simulated. The perfect echo data of spot target is generated, and then the images of the spot target processed with the R-D algorithm are used to verify the feasibility of processing program.