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The focusing performances of two-step algorithm are analyzed in the paper. The convolution processing in two-step algorithm is detailed analyzed. It can be found that the operation can reduce the Doppler bandwidth effectively and overcome the alias of azimuth echo by making the residual Doppler bandwidth smaller than PRF. The formula of residual Doppler bandwidth is derived here. According to the formula, range large bandwidth signal has influence on the residual Doppler bandwidth, which is not considered in the traditional imaging algorithm. In the high resolution sliding spotlight space-borne SAR system, the residual Doppler bandwidth is affected largely by the wide range bandwidth. PRF should be larger than the residual Doppler bandwidth in two-step algorithm; else it will result in defocus of the SAR image. The computer simulation is given to verify the presented analysis.