Abstract: In order to acquire a more exact surface figure error result,the method about solving the Ritchey-Common test data was studied. Based on the existing mathematical model, a method which utilized the relationship between the system pupil coordinate and test flat mirror surface coordinate to recover the surface error condition was introduced. The effectiveness of this method was verified by simulation, the theoretical accuracy of this method was also analyzed. Comparing the test result with the result measured by interferometer directly, we can know the actual test accuracy of coordinates transforming method: the peak-valley(PV)value is better than 1/20 ,the root-mean-square (RMS) value is better than 1/100 , which can satisfy the high requirement of test. Even comparing to result of impact matrix method, the accuracy of PV and RMS can improve 0.013 ,0.0037 respectively .This indicates that the coordinate transforming method is more suitable for Ritchey-Common test.