A comprehensive and novel SAR echo simulation method for a great deal of scattering cells is presented in this paper. In order to greatly reduce the processing time, we apply the Graphics Processing Units (GPU). Modern GPUs are very efficient at manipulating computer graphics, and their highly parallel structure makes them more effective than general-purpose CPUs in processing large blocks of data. Especially when facing the complicated scene of many scattering cells, high-speed operation in parallel based on GPU is perfectly fit. Via Compute Unified Device Architecture (CUDA), each thread of GPU is in charge of a single scattering cell, and thus this structure will highly improve the efficiency of echo simulation in complex scene. (5 pages)