Multi-channel Azimuth SAR system can overcome the " minimum antenna area" constraint" and achieve high resolution and wide swath SAR imaging. As each receive channel is independent of each other, it is difficult to avoid phase biases between the channels, which leads to the degradation of system performance, such as increased ambiguities. To solve this problem, a novel channel phase bias calibration algorithm suitable for Multi-channel Azimuth SAR system is proposed in this letter. Theoretical analysis reveals that the phase biases between channels can result in the decrease of image contrast. So, the contrast of sub-aperture image is taken as the optimization objective. By maximizing the contrast of sub-aperture image, the phase bias between the channels can be precisely derived. The effectiveness of the proposed algorithm is validated by experimental results.