Abstract: In order to solve the contradiction between the wavelength and transmit power of Ka-band spaceborne Synthetic Aperture Radar(SAR), the traditional always use the DBF-Scan-On-Receive(DBF-SCORE) algorithm to improve the antenna gain. However, in the undulating terrain, the pre-set weight vector will cause serious deviation of the beam pointing and the loss of the receive gain, which is called the elevation mismatch. To solve these problems, based on the spatial spectrum estimation technique, this paper studies an adaptive digital beam forming algorithm which is suitable for Ka-band SAR. This algorithm can accurately estimate the Direct of Arrival (DOA) of the return wave and optimize the receive gain. This algorithm improves the gain loss of the traditional algorithm caused by the elevation mismatch. The simulation analysis proved the algorithm is stable while the environment system parameters changes.