A calibration method for the cross-angle between the optical axis of a remote sensing camera and that of a star sensor was investigated to improve the positioning accuracy of remote sensing images. On the basis of the rigorous space resection model, the unified calibration method of interior and exterior orientation elements was proposed by introducing an improved non-direction solving model into the calibration of mounting error of the remote sensing camera. By which, the system error between attitude determining sensor and remote sensing camera was compensated accurately. In processing the remote sensing images from a satellite, the relationship matrix between remote sensing camera and star sensor attitude were determined by the optimized model, then the cross-angle was calibrated accurately. Finally, the cross-angle model was verified with the geometrical relationship of single-image targeting. After the calibration, the plane position RMS errors are 9.31 m and 9.28 m in latitude and longitude directions, respectively, which shows that the positioning accuracy of the remote sensing images have been improved greatly by the proposed method.