The Ronchi grating is used to test the large error of aspheric surfaces during the grinding and advance polishing combining with the technology of photo sensors and the technology of digital image processing in the quantitative way. This paper discusses the basic principle of this method, sets up the mathematical model and works out the corresponding software. A concave parabolic mirror with the diameter of 140 mm and the F number is 2 is tested by using this method. The result of the quantitative measurement fits well with the one of the interferometer. This analysis software and test method provide a valid and credible quantitative measurement way for the exact-grinding and advance polishing aspheric surfaces.