Based on the spaceborne parasitic SAR earth observation application, the author aimed to analyse the influence of the frequency synchronization error to the bistatic SAR imaging. The “interferometric cartwheel” configuration was employed as an example, to make computer simulation of the synchronization error. Therefore, a novel frequency synchronization method was presented. This method acquires the frequency synchronization error by the correlation processing to the “direct signal” which leaked from the SAR antenna side lobe. The simulation results indicate this new synchronization method is accurate and effective.