To analyze key events of complex systems, the event sequence diagram is translated to an influence network. By colored petri net (CPN) modeling and simulation, the influence network is used to analyze sensitivity of the system events and search key event sets. According to the output of the CPN model, conditional probability and baseline probability are obtained statistically. The relationship between causal strength and conditional probability is linearized to obtain the causal strength value objectively. We apply the proposed method in a ballistic missile defense system. The results show that the method is straightforward and practical. It is useful in decision-making of complex simulation systems.