The tropospheric freezing model assumed in the low earth orbit SAR (LEO SAR) is no longer valid in the extremely long synthetic aperture time of the Moon-based Synthetic Aperture Radars (SAR), since the tropospheric path delay based on the kinematic model is not suitable for the curved trajectory the Moon-based SAR. In this paper, we examine the effects of the temporally varying tropospheric path delay on the L-band Moon-based SAR imaging by establishing a proper signal model. Theoretical analysis shows that both the range and azimuth shifts are profound for the Moon-based SAR. As for image quality, the range focusing is not disturbed by tropospheric path delay, whereas the azimuth imaging is severely defocused with a synthetic aperture time longer than 500 seconds. Such geometric deviations and image distortions apparently should be compensated for the Moon-based SAR system in terms of image quality.