Abstract: TOPSAR can greatly improve the efficiency of SAR mapping through fast beam scanning. However, the beam of the traditional TOPSAR mode scans only in one side of the radar platform. To further improve the efficiency of SAR mapping, an improved TOPS model, which is named as bilateral TOPSAR mode is proposed. In this new mode, the radar beam scans at both sides of the radar platform in sequence. For this new SAR mode,an image formation method which is based on subaperture processing is proposed. The new approach produces subaperture images and mosaics them to get a large scene image.It can produce images when antenna beam is scanning. The results of the simulation and real data show the effectiveness of the proposed algorithm.