Abstract: Aiming at the complex in the process of network security risk assessment, the asset, vulnerability and threat were used as the major factors in security assessment to establish the hierarchical index system for security assessment. The concept of credibility was introduced, and the security risk assessment model and fuzzy reasoning algorithm based on fuzzy Petri net were also proposed, making use of fuzzy Petri nets method joined together with the AHP to analyze the question, and combining qualitative analysis and quantitative analysis together. The example analysis shows that the obtained results are more accurate and scientific compared with traditional assessment methods. Therefore, this method is an effective network system risk assessment method.