Segmented mirror method is widely adopted in large optical telescopes nowadays in the world. Using segmented mirrors to make up a large mirror requires the segmented mirrors co-focus. It needs corresponding co-focus detecting methods to measure the co-focus errors. In China this kind of research is insufficient nowadays. In the paper, Zernike polynomial is used as Optical-Mechanical Interface, with the help of the engineering softwares, such as UG, Patran, Matlab, and the optical software CODE V to model and simulate the widely used Shack-Hartmann method. The relationship between S-H measurement and segmented mirror co-focus is found. Finally, a computer aided design method for optical measuring of segmented mirror co-focus is proposed.