The method of Program Evaluation and Review Technique (PERT) has been applied to the field of project management extensively. However, traditional PERT is still defective when used in schedule control. According to the principles of PERT and the hypothesis that the activity duration in PERT obeys Beta distribution, this paper points out the disadvantages of traditional method in determining critical path in PERT network program, proposes the concept of "Critical Degree" to describe the importance degree of paths in PERT network, then puts forward Monte Carlo Simulation (MCS) to analyze project schedule uncertainty. The Example shows that MCS can overcome disadvantages of the traditional PERT method. At last, through the sensitivity analysis of each critical activity, this paper points out the critical activities that should be selectively controlled.