Raw signal simulation is considered the base for the study of distributed small satellite SAR system. Huge computation is needed when raw signal of wide area natural scenes is simulated. A rapid algorithm using FFT is proposed to reduce computation. Firstly, time domain response of the natural scene is approximately expressed by pulse sequence. Then in frequence domain, LTI filter is realized with FFT to generate raw signal. The results of SAR imaging and interferometric processing of simulated raw signal validate the algorithm.