Abstract: Geosynchronous SAR tomography (GEO TomoSAR) is a technique that combines geosynchronous SAR (GEO SAR) with SAR tomography. It can overcome the shortcomings of long revisit time and serious decorrelation in low Earth orbit SAR tomography, and realize timely and accurate three-dimensional reconstruction of targets. However, due to the high orbit of GEO SAR, the signal can pass through the entire ionosphere, causing the signal to have a long time delay. Signal delay introduces the error phase, which in turn affects the accuracy of GEO TomoSAR 3D imaging. In this paper, based on the influence mechanism of background ionosphere on radar signals, the GEO TomoSAR signal model under the influence of temporal-spatial variant background ionosphere is established, and its influence on three-dimensional imaging is analyzed. After analysis, the influence of the background ionosphere on the GEO TomoSAR system mainly includes the relative offset of the scattering target and the imaging defocus in the elevation direction. Finally, the correctness of theoretical analysis is verified by computer simulation.