Abstract: Terrain Observation by Progressive scans SAR (TOPSAR) is a novel spaceborne imaging mode with wide swath coverage. Imaging algorithms for such mode are required to resolve three existing problems: Doppler spectrum aliasing, large range cell migration and azimuth output time folding. As for these issue, a new imaging algorithm based on two-dimension Chirp-Z transform is proposed. The complete derivation processing of the presented algorithm and the expression of each transfer function are given in detail. In this algorithm, azimuth pre-filtering with deramp operation is adopted to resolve Doppler spectrum aliasing problem with fewer azimuth samples compared with other methods. Chirp-Z transforms in the range and azimuth domain can implement the large Range Cell Migration Correction (RCMC) and azimuth data focusing, respectively. The presented imaging algorithm only requires the limited azimuth increased samples and with interpolation free. Therefore, it is with high computational efficiency. Simulation results validate the effectiveness of the presented imaging algorithm.