The Medium-Earth-Orbit SAR (MEOSAR) is one of the potential spaceborne SAR of next-generation owing to its excellent performance. Ionospheric effects analysis is one of the critical techniques for the development of MEOSAR. The spatio-temporal variability of the ionosphere is analyzed using the measured ionosphere data. The factors and the mechanism of ionospheric effects on MEOSAR are studied based on the spatio-temporal variability of the ionosphere and the characteristics of MEOSAR such as long synthetic aperture time, wide swath, and high orbit. The results reveal that there are many differences in the ionospheric effects between MEOSAR and LEOSAR.