Abstract: Time domain synthetic aperture imaging algorithm can adapt the non-uniform sampling on azimuth direction caused by multi-element arrays,and it has the advantages of lower memory demand and convenient for parallel processing.The computing load of the exact time domain algorithm is huge.However,using FFBP imaging method can reduce the computing load dramatically.Detailed analysis is made including the range effect of error,sub apertures combination and image factorizing strategy,and computing load of FFBP imaging algorithm.Simulation and real data results show that FFBP algorithm is a proper choice for real-time SAS system due to high computation efficiency and good quality of SAS imaging.