Abstract: Medium-orbit SAR (MEO SAR) is an important complement to low-orbit SAR and geosynchronous orbit SAR because of its advantages of wide swath and short revisit time. However, compared with low-orbit SAR, MEO SAR has the characteristics of long synthetic aperture time, complex orbital characteristics. Conventional echo model is not suitable for the imaging of moving targets in MEO SAR. In this paper, the slant-range models of MEO SAR are quantitatively analyzed. It is concluded that the fourth-order slant-range model can meet the imaging requirements when the ship is in the case of static, uniform motion and acceleration. But it will be invalid when the ship is affected by ocean waves. Finally, the accurate echo model of medium-orbit SAR is realized.