The mathematical model of probabilistic optimization is established for diaphragm spring. The process and the method of design with an example are presented. The result shows the model and the method of design are feasible, and perfect optimal result is attained. Vehicle has mass production and its conditions of use are changeable, which determine design parameters and operating parameters of its parts have complex randomness. Diaphragm spring clutch is now widely used by many kinds of vehicles. Research of diaphragm spring parameters’ probabilistic and statistics model and the study of its optimal design have great significance for the design, R & D, manufacturing and use of automotive products and parts. Studies in the references[1,2,3] have shown that the root cause of diaphragm spring’s fatigue fracture is the original crackles near the concave inner edge point, so there is a need to do probabilistic optimal design for diaphragm spring’s structural parameters to improve the fatigue life.