Abstract: Due to the high orbit and long synthetic aperture time of Geosynchronous SAR (Geo-SAR), the error brought by the Stop-Go assumption has to be considered. In this paper, an accurate signal propagation delay time equation of Geo-SAR based on circle orbit is presented, and the error of Stop-Go assumption is shown by simulation. An equivalent position model is proposed, and is validated through simulation. Error of different powers of Stop-Go slant range is analyzed using Legendre orthogonal series expansion. The limitation of Stop-Go assumption and the availability of Equivalent Position mode are proved by point target simulation.