Abstract: This paper starts from the expression of frequency modulated continuous wave synthetic aperture radar (FMCW SAR) echo signal. Firstly, the characteristic of FMCW SAR echo isanalyzed, and then the accurate signal model and instantaneous spectrum are obtained. Based on these, the equivalent Dechirp signal model is obtained. Secondly, the signal model of FMCW SAR and pulse SAR are compared, and then the influence on the echo owing to the successive motion within a pulse sweep is revealed, based on which, the impact on the image reconstruction is quantitatively analyzed. At last, whether the "stop-go-stop" approximation is valid in FMCW SAR is revealed. The simulation results verify the effectiveness and exactness of the above analysi.