In traditional design of an optical system, optical design and mechanical design is relatively independent. It′s difficult for a mechanical designer to know which structure is suitable for an optical system, and even harder to optimize the mechanical structure. The mechanical sensitivity is utilized to build the relationship between the caused error of the mechanical structure which includes manufacture and assemble and the quality of the optical system, and it′s easy to see how the mechanical manufacture and assemble errors affect the displace-ment, tilt and space errors and then affect the optical system quality. Mechanical designers use the mechanical sensitivity table of different mechanical structure to decide which is the best, and optimize the opto-mechanical structure.