Airborne Multiple-Input Multiple-out (MIMO) SAR system can be used for achieving the image with high resolution, but the inevitable problem of motion compensation must be solved. The MIMO-SAR system with several sub-bands is studied here. Firstly, a model of motion error for MIMO-SAR system is established and a detailed analysis of it is presented. Then an extended MIMO-SAR Range Migration Algorithm (RMA) integrating with motion compensation is proposed. Range Cell Migration Correction (RCMC) and azimuth focus are separated by improved Stolt-mapping, and the motion error is corrected through two steps of compensation. Finally, sub-bands are combined into a whole band in spatial-frequency domain to obtain a high range resolution. Imaging simulations for point-targets and plane-targets with this algorithm are performed to verify the validity of such algorithm.