Aiming at geosynchronous synthetic aperture radar (GEOSAR), there is an experimental study on phase calibration caused by ionospheric disturbances, this paper proposes a method of measuring ionospheric total electron content(TEC) based on peak sidelobe ratio. Firstly, analyze the ionospheric affect on GEOSAR experimental signal, after signal range compression, calculate the peak sidelobe ratio. Then, estimate quadratic phase error caused by the ionosphere and inverse the ionosphere TEC. Finally, the simulation analysis prove the feasibility and applicability of this method. The conclusion is that this method is suitable for the GEOSAR experimental study on phase calibration caused by ionospheric disturbances, and the estimated TEC in the low band (such as Pband) is closer to true value.