Synthetic aperture radar echo signal is the foundation of SAR imaging and other processes, the analysis of SAR signal spectrum is helpful to the design of the pulse compression that is to say the design of the matched filter,the SAR signal spectrum is the theoretical basis of some accurate and efficient imaging algorithm, such as RDA,wKA,CSA.Through the analysis of the spectrum,some problems like range cell migration,doppler ambiguity in SAR imaging will be solved more easily.This essay focuses on the analysis and derivation of the SAR signal in the range doppler domain and two dimensional frequency domain to get the spectrum expressionand profoundly understand the essence of the Fourier transform; Finally, by Matlabsimulating the characteristicsof the receivedsignalonasingle point target under zero squint angle and nonzero angle respectively,analysing the characteristics of SAR signal spectrum properties in these two domains theoretically and experimentally.