Abstract: The paper studies ionospheric scintillation impacts on GEO SAR focusing based on multiple phase screen (MPS) simulations. The power spectrum of electron irregularities is modified according to the anisotropic ionosphere. In the case of the oblique incidence, the electromagnetic wave equations in random medium are derived, and then the MPS simulations are realized. After the simulation of GEO SAR imaging, the effects of the random fluctuations are analyzed. Finally, the optimized integration time under different scintillation conditions are proposed through Monte Carlo simulations. The paper concludes that the greater ionospheric fluctuations and longer integration time will lead to more serious deterioration in the same GEO SAR configuration; and even not focus at all for the worst case.