The scan law of system using 45° mirror and its effect on correction of Image rotation was analyzed. The result of the correction of image rotation by software on ground was illustrated. After concerning the radius of the earth and the off-axial and mounting angel of the detector, it turns out that the system can acquire thorough ground information when increasing scanning angle, and the image rotation does not influence the space resolution and the band match of the system ,and image rotation can be corrected by software.