A distortion correction method based on ray tracing of test optics is proposed. The position of the symmetric distortion center and the sampling interval of the test wavefront are solved by extraction and analysis of the surface boundary, then the relationship between the mirror under test and the surface map is established by ray tracing to correct the distortion. The 730 mm ×268 mm primary mirror of an off-axis threemirror optical system is tested by null lens, and this method is applied to correcting the distortion of the surface maps. After correction the maximum of the standard correlation function between the binary map and the binary mirror image increases from 0.925 to 0.985, the improvement is conspicuity.