Small size, light weight, and low power are presently the directions in SAR development. The microSAR platform is small and light, which results in track deviations because of air flow. The large motion error strongly affects the quality of SAR images. Therefore, high-precision motion compensation is important to SAR image processing. Motion error results in phase and envelope errors. Traditional motion compensation algorithms often ignore the space variance of the envelope error. When the motion error is large, the space-variant envelope error affects the image quality. This study proposes a high-precision motion compensation method based on the subaperture envelope error correction for SAR. The proposed method minimizes the effect of the space-variant envelope error and improves the image quality. Simulations and experimental data processing validate the correctness and effectiveness of the proposed algorithm.