Abstract: The Medium-Earth-Orbit SAR (MEOSAR) is one of the potential next-generation spaceborne SARs. Ionospheric effects analysis is one of the critical techniques for the development of MEOSAR. An analysis model for background ionospheric effect on MEOSAR is established based on the system characteristics of MEOSAR and the spatio-temporal variability of ionosphere. The degradation of image quality, including resolution and displacement distortion, induced by background ionosphere and its spatio-temporal variability is analyzed. The analysis result shows that ionosphere and its time-space variability affect critically the quality of obtained images. In condition of the same ionosphere, the degradation of resolution in both azimuth and range, the distortion on range image and the displacements in azimuth image are all more serious with the increasing of SAR orbit height when the resolution is same.