Abstract: The Estimation of spaceborne SAR Doppler frequency rate is studied. A polynomial model is set up to demonstrate the azimuth signal of the spaceborne SAR and this method converts frequency rate estimation to frequency estimation through difference processing. With the analysis of estimation accuracy and computational complexity, the method is proved to be able to reduce the burden of computation while the estimation accuracy is guaranteed. Finally, the raw data of RADARSAT are used to test the method and the estimation results are given to compute the image of raw data which proves the effectiveness and feasibility of the method. This method can be used on quick look digital image processing of spaceborne SAR.