Frequency Modulated Continuous Wave (FMCW) Synthetic Aperture Radar (SAR) is a small-size, light-weight and cost-effective imaging radar, which is suitable for operation on a small flight platform. In the dechirp processing of FMCW SAR, the transmitting signal is more feasible for using as the mixing signal. But the bandwidth is decreased because of echo delay in the dechirp processing. The bandwidth decrease becomes serious when the target is far away or the pulse repetition time is short, and the range resolution is reduced. We constructed a unified compensation function to compensate for the echo signal, thus the bandwidth was fully exploited and the range resolution was unaffected by echo delay. Simulation results prove the correctness of the compensation function and the effectiveness of the compensation method.