The iterative positioning method (IPM) based on digital elevation model (DEM) is an important way in object positioning or map revision with single remote sensing image. The geometry principle of synthetic aperture radar (SAR)- IPM has not been clear so far. In this study, the iterative calculation process of SAR-IPM was analyzed by using geometry theory based on distant projection of SAR image. Two known variables, slope obliquity and incidence angle of initial position, were chosen to derive the approximate convergence criterion and the convergence speed function. It is found that slope obliquity and incidence angle are leading factors for the convergence and convergence speed of SAR-IPM. Experiment results with simulated data and real data have proved that the approximate convergence criterion is accurate and reasonable, and the convergence speed function is correct. The result will help to analyze the geometry relation between the SAR image point and the corresponding ground point.