Spaceborne multi-channel SAR is an effective means to achieve high-resolution and wide-swath imaging. Due to the signal undersampling, the unambiguous spectrum reconstruction should be conducted firstly. However, the channel imbalance will cause the failure of reconstruction. For moving targets, the motion will result in inter-channel phase error. Different motion speeds correspond to different phase errors, and hence the existing methods which estimate a uniform phase error fail to estimate the phase errors of each target. To solve the problem, the paper analyses the phase error caused by target motion and proposes a multi-channel SAR imaging algorithm for moving targets. By estimate the phase errors of moving targets separately, the phase errors can be compensated accurately. After compensating the inter-channel phase errors, the accurate imaging can be conducted to obtain a well-focusing SAR image without azimuth ambiguity. The effectiveness of the proposed algorithm is verified by a computer simulation.