As a high-precision testing method for optical surfaces, accuracy of the subaperture stitching interferometry is very important and must be evaluated quantitatively. The contrast test approach is proposed to evaluate the measurement accuracy with the full aperture test taken as a reference. Then the figures of merit as well as their computing methods are discussed. Since the geometrical parameters are different in the full aperture test and the subaperture stitching test, optimal matching is proposed between the two test results. The principle resembles that of the subaperture stitching algorithm. The figures of merit are computed following the matching process. Finally experiments are presented, and the results show the figure of merit with matching is reduced by about 48% of that without matching, which proves the validity of the approach for quantitative accuracy evaluation of subaperture stitching interferometry.