For synthetic aperture radar (SAR), range ambiguity causes a great deterioration in imaging performance. To suppress range ambiguity, the azimuth phase coding (APC) technique stands out for its effectiveness with a low implementation complexity among the available approaches. With proper phase modulation and demodulation, the position of an ambiguous signal is shifted in Doppler spectrum and then part of the ambiguity can be filtered out by an azimuth filter. However, since the suppression performance heavily depends on the system oversampling rate, the APC technique cannot achieve the same suppression performance for a multichannel SAR system compared with a single-channel SAR system. A method to suppress the range ambiguity for multiple-input, multiple-output (MIMO) SAR system based on APC technique is presented. By taking advantage of more phase centers of the MIMO SAR, a proper azimuth beamformer weight vector can be computed to null out the ambiguity position in the azimuth frequency domain and reconstruct the useful signal; thus most of the ambiguity components can be significantly suppressed. Finally, the simulation results validate the effectiveness of the proposed method.