In order to improve the accuracy of Ritchey-Common (RC) method, the effect of accuracy of Ritchey angle on test results is mainly discussed. By means of simulations, the measured accuracy of surface error can reach 0.01λ(λ=0.6328 μm) when Ritchey angle′s range is optimized between 20°~50°. The error of Ritchey angle is simulated to analyze the influence on measured result. When the error of Ritchey angle is controlled within ±1°, the accuracy of residual error between fitting results and original surface can be decreased to 0.0007λ and meets the measured requirement. To avoid the measurement error of Ritchey angle, the ratio of image size on pupil plane is used to calculate the Ritchey angle, and this method′s accuracy can reach to 0.2°. Three Ritchey angles are chosen in the experiment and combined with each two of them. For the residual error between Zygo measurement and group of 29.6° & 47.8° measurement, the peak valley (PV) is 0.068λ and the root mean square (RMS) value is 0.0105λ. The results demonstrate that the choice of Ritchey angle and its calculation accuracy are important to RC method.