As a key part of diaphragm spring clutch, diaphragm spring determines the load characteristics of the clutch by its load-distortion properties that are sensitive to the dimension of diaphragm spring. In order to predict or determine the probability of normal performance under desired working condition to control qualified rate of products, calculus of probability model for load-distortion properties of diaphragm spring by means of mathematical statistics was established. Based on the model, the standard deviation was calculated by applying normal distribution algebra, first order second moment method and Monte-Carlo simulation. The standard deviation is relatively less by normal distribution algebra; bigger by first order second moment method, and fluctuant along with false random sample by Monte-Carlo simulation. These three methods all satisfied design precision of automobile clutch diaphragm spring. 