Recently, circular synthetic aperture radar (CSAR) has attracted much attention in the field of high resolution SAR imaging. However, the focusing of CSAR image is affected by the precision of digital elevation model (DEM), which is used for generating imaging grid. In this paper, the effect, which is due to limited precision of DEM, is given in detail. And the method based on subaperture process is proposed, and the additional condition for application is given. The real CSAR data processing results prove the validity of the theoretical analysis and the proposed method.