In this paper, a new high-precision electromagnetic scattering computing model, which is based on the parallel finite-difference time-domain (FDTD) method, is proposed to compute the SAR echo. Since the FDTD method can only compute radar return at a fixed azimuth angle once, several FDTD simulations should be combined so that a flight path can be simulated. The FDTD method suffers from low computation speed and large computation cost, thus a parallel FDTD model based on MPI library is built to overcome these shortcomings in this paper. The purpose of the computing model is how to use the parallel FDTD to get high-resolution SAR echoes rather than imaging techniques or electromagnetic propagation and scattering effects. Then the SAR echoes of some typical objects including tank and car are computing. To validate the correctness of the proposed model, SAR images derived from the computed radar echoes are given in this paper. To evaluate the performance of the proposed method, experiments are done with different number of threads and PCs. The results show that big improvement has been made in speedup and computational scale by the proposed model.