The Ritchey-Common method is one of the main methods to measure the large-aperture flat mirror with small-aperture interferometer. By analyzing the influence of radius of spherical reference mirror, position of flat mirror and error of Ritchey angle on the measurement accuracy in this method, the selection method of the three-parameter was given. Aiming at the inefficient measurement caused by error separation in Ritchey-Common method, a rapid Ritchey-Common measurement method during processing was proposed. The influence of positioning errors such as position and angle of the measured flat mirror on the measurement results was simulated, and the experiments were carried out. The maximum deviation peak-valley value (PV) between this method and the direct measurement method is 0.015 1 μm, and the root-mean-square (RMS) is 0.003 6 μm, which can effectively meet the needs of rapid surface measurement in processing.