In order to realize high precision Ritchey-Common test for large flat mirrors, the relationship between the system pupil coordinate and test flat mirror surface coordinate is utilized to dispose the test wavefront, and then combine with the least square method to detach the alignment errors which are caused by the adjustments of optical system, after this an exact flat surface can be obtained. It analyzes the effects of the test distance on the relationship between the two coordinates and the value of Ritchey angle, and makes the test program according to analysis. In the test process, two measurements in different angles have been taken and the wavefront is found, after detaching the power errors, the peak-valley (PV) and root mean square (RMS) of the final results are 0.182λ and 0.0101λ, compared with the results of Zygo which are 0.229λ and 0.013λ for PV and RMS, the test accuracies of PV and RMS can reach λ/20 and λ/100, respectively. The experimental results prove that this method is effective to calculate the test flat surface and the theory of accuracy analysis is correct, it also realizes the high accuracy of Ritchey-Common test.