In order to improve the accuracy of an off-axis reflected space optical system, researches have been made to its distortion feature. In the research, some distortion features different from the radial distortion model are found. So the previous solutions are no longer suitable for this problem. A method using two-dimension Lagrange interpolation is introduced to correct the distortion of the off-axis system. As the system is still in design, these methods are verified and analyzed by simulating in computer. The results show that by using this method, the distortion of the off-axis system can be well corrected.