Failure probability calculation is what the large dam risk evaluation is based on.The computer simulation method can solve the difficulty in linearizing the state function

normalizing the nonnormal random variable equivalent and transforming the relevant random variables during the calculation of dam failure probability by means of First Order Second Moment method.On the basis of the analysis of the major dam failure modes

a probe is made into the computer simulation method and its procedures for direct calculation of the failure probability;while considering the dam shear-slipping collapse mode

a study is made of the state function and the distribution regularities of the random variables under this mode and the formula is deduced out for generating the random numbers for random variables of Pierson III Distribution

Extreme-value I Distribution and Logarithmic Normal Distribution;by programming

repeated simulations are conducted to the dam shear-slipping failure mode and through statistics and analysis of the results of the simulation

the mean value

the standard deviation and a 95% confidence interval of the shear-slipping failure probability are obtained.