The basic method to improve efficiency of the BP(Back-Projection) algorithm is the sub-aperture’s way.Choosing the length of sub-apertures and the resolution of sub-images becomes the most crucial step that decides the computation efficiency.The condition that limits the image resolution is analyzed taking the bandwidth and the range error into account.Then a same result presenting the relationship between the sub-aperture length and the resolution is received.A method for dividing the sub-aperture and choosing the image resolution in UWB signal’s circumstance is presented based on the former result.Besides the Multi-Stage Multi-Resolution Back Projection (MSMRBP) algorithm is presented to suit the nonuniform aperture condition.At last the outside experiment’s results are presented to support the former conclusion.