GEOsynchronous Circular SAR (GEOCSAR), having a high orbit, operating in a slant ellipse, and imaging in spherical surface, should be newly modeled to assess the resolution performance. In this paper, starting from modeling geometry, a function of slant range difference and orientation angle is deduced. Then we can obtain the three dimensional point spread function and make a conclusion of resolution affected by both sinc function and bessel function of first kind. Lastly, resolution with the variation of location of target, bandwidth, radius of orbit, and integral time is analyzed detailedly. Finally raw signal simulations show the validity of theoretical model.