The SAR imaging system is timeconsuming, which is developed based on central processing unit (CPU), because of the huge computation of SAR image processing, making realtime imaging impossible. Based on Computed Unified Device Architecture, This study proposes a new plan for realtime SAR imaging operated on graphic processing unit (GPU) which works for multiple workingmode SAR. This new proposal makes the plan work with small GPU memory by dividing SAR data into pieces, uses the asynchronous execution of multistream technology to cut down the time wasting in data transfer, optimizes memory management to get less access time of memory and uses lightweight function of special function unit (SFU) to reduce clock cycle. All computational resources are totally exploited because this plan is suitable for multi GPU. It has been shown by an experiment on an NVIDIA GT740M and INTEL Q6600 that the proposed plan has an acceleration ratio of nearly 150. The proposed realtime SAR imaging system improves the imaging efficiency greatly and has a better application future.