To realize the high-precision distortion analysis and measurement of an off-axis optical system and fast and high-precision distortion correction of on-orbit images， a high-order distortion analysis method is used to fit and analyze the relative distortion of the off-axis optical system. First， the theoretical distortion of the off-axis optical system is examined. Then， the concept of equivalent focal length is proposed， and the relative distortion of the system is fitted and analyzed using the aforementioned analysis method. The distortion testing and focal length measurement are then completed for the well-aligned off-axis optical system. Finally， the measured results are used to complete the object image correspondence of the off-axis optical system. The maximum deviation of the equivalent focal length calculated using the high-order distortion analysis method is 0.366 mm when compared with the results of the ZEMAX ray tracing. The measured focal length deviation of the equivalent coaxial system for an off-axis optical system is only 6.378 mm， and the relative deviation is 0.073%. The correctness and accuracy of this method for distortion analysis and measurement and focal length evaluation of the off-axis optical system are verified， thereby providing reliable input for the fast and high-precision distortion correction of on-orbit images.