Abstract: The frequency synchronization errors exist in the Local Oscillating Signals of the transmitters and receivers of the passive SAR based on spaceborne transmitters because the transmitters and receivers are mounted on separated platforms. Phase errors is introduced in echo demodulation process because of the frequency synchronization error, and then, the final focus result will be deteriorated. This paper focus on the passive fixedreceiver SAR based on spaceborne transmitters. First, this paper discusses the influence of various frequency synchronization errors on azimuth phase. And then, a frequency synchronization approach of using direct-path echo is presented, in which we obtain the phase error from direct-path. In the process of obtaining the phase error, the Zero Doppler Time is the key. A more precise estimation method of the Zero Doppler Time has been improved, and the frequency synchronization is achieved. Finally, simulations are carried out to prove the validity of the research result.