Abstract: This paper presents a method of network security situation assessment based on membership cloud model. Firstly, a multi-level and multi-dimensional extensible network security evaluation index system were built in terms of transport services, assets, vulnerabilities, security events and physical equipments, and an index pretreatment method was designed for each dimension. Secondly, a three-layer model framework which included the data layer, the information layer and the knowledge layer was designed, and the index standardization and weight calculation method, the security situation calculation process were given, and the network security situation assessment algorithm was designed. Finally, we designed the network security situation assessment platform and simulated the experiment on the platform. The experimental results show that the effectiveness and rationality of our method is fine.