BP（Back-Projection）algorithm is a popular imaging algorithm of SAR.Although it can compensate the range migration which is inevitable in SAR imaging problem with wide processing-angle perfectly,its great computational burden makes it the bottle-neck in speeding up the signal-processing rate.By employing the characteristic of radar's track in Rail SAR,this paper removes the redundancy in BP algorithm to increase imaging rate greatly.This algorithm has achieved good effect in the experiment.