Abstract: The channel imbalances will reduce the performances of the multi-channel synthetic-bandwidth SAR system, so measures should be taken to correct them. In this paper, a correction method based on the raw data is presented. Before acquiring the synthetic-bandwidth signals, the system is switched to emit and receive single sub-band signals, and then the channel imbalances are extracted from the Doppler spectra between different channels. In the method, the additional inner calibration circuits can be left out, which will simplify the system design. Finally, the practicability of this method is confirmed by the processing of the real data which obtained by the displaced phase center multiple-beam SAR system.