Abstract: Compared with low orbit Synthetic aperture radar (SAR), geosynchronous Earth orbit Synthetic aperture radar (GEOSAR) has more advantages, such as wider swath, shorter revisiting period, so has a wide range of civil and military applications. Due to the satellite timevarying Doppler centroid and analogous sliding spotlight work mode, conventional azimuth ambiguity calculation method is not accurate when used in geosynchronous Earth orbit Synthetic aperture radar. This paper proposes a new method for analyzing the azimuth ambiguity. Based on accurate satellite-earth geometry model, the Earth's rotation, timevarying velocity, time-varying Doppler centroid and antenna pointing variance in the analogous sliding spotlight work are considered in the new method. The azimuth ambiguity is obtained accurately when the antenna gains including no ambiguity area and ambiguity area are both calculated. And then the expression of GEOSAR azimuth ambiguity was deduced based on spatial coordinate’s transformation and vector representation. Finally, simulations are carried out to verify the proposed approach using a set of GEOSAR parameters.