Abstract: The absolute test technology of optical surface shape avoids the limitation of reference surface shape accuracy of interferometer and can effectively improve the detection accuracy of flat-crystal surface shape. The absolute test technology of two-flat crystal three-sided mutual test of N-position image rotation method was adopted to solve the three-dimensional absolute surface shape distribution of tested flat-crystal, and the results included the information in medium frequency band. The recursion formula was used to construct the N-position virtual rotation results of rotational change item. After summing and averaging, the rotational change item was obtained, and the tested wave surface shape was obtained after superposition of the rotational invariant item. The theoretical error of the algorithm was derived, the rotation angle was optimized and the number of virtual rotation was increased, and the accuracy of the algorithm was improved. After optimization, the simulation results show that the RMS value accuracy of residual wave surface is 0.14 nm. The two-flat crystal three-sided mutual test of flat-crystal with 150 mm caliber was carried out, and the experimental results were compared with those of the traditional three-sided mutual test method. The RMS value deviation is less than 0.5 nm, which verifies the accuracy of the algorithm.