We analyze the error caused by `stop-and-go' assumption on Moon-based SAR image focusing. In Moon-based SAR imaging of earth, the extremely long propagation time delay and curved trajectory violate the `stop-and-go' assumption. The separation of transmitting and receiving antenna in Moon-based SAR is not regarded as monostatic, but quasi-bistatic mode. The Doppler parameters should be estimated under this condition. Preliminary analysis shows that error caused by stop-and-go assumption can seriously affect the focusing of Moon-based SAR imaging, and should be corrected or compensated.