Image quality analysis by CODE V software can obtain more accurate result of a main-secod mirror optical system based on each surface′s real wavefront aberration of surface error, and this result can be used to direct the alignment of the system. A 520 mm diameter RC system is analyzed, and the result is compared with real interferogram of surface error and non. It is shown that the former result is more close to the real wavefront aberration tested by a interferometer, which proves the feasibility and reliability of the proposed method used in optical alignment engineering.