Abstract: This paper introduces the shortage of the traditional tolerance design method and the strongpoint of the computer aided tolerancing （CAT） method,compares the shortage and the strongpoint among the three tolerance analysis methods,which are Worst Case Analysis,Statistical Analysis and Monte-Carlo Simulation Analysis,and offers the basic courses and correlative details for CAT.As the study objects,the reactor pressure vessel,the core barrel,the hold-down barrel and the support plate are used to upbuild the tolerance simulation model,based on their 3D design models.Then the tolerance simulation analysis has been conducted and the scheme of the tolerance distribution is optimized based on the analysis results.