Abstract: The reactor zone of QinshanNaclear Power station in its third engineering stage is the research object in the paper. A random approach was taken to study the distribution regularities of joint cracks in rock masses due to their random characteristics. According to the attitude, spacing and length of the joint cracks, their probability distribution was deduced. Heterogeneity and orientation of the joint cracks were considered fully in this method. A non steady stochastic process is used to simulate the joint spacing, and an artificial joint network is generated by using Monte Carlo method. From the results of statistics and simulation, the joint cracks in rock masses in the reactor zone was assessed.