Abstract: A network security evaluation model based on Finite State Automata (FSA) is proposed.According to all kinds of security information and its related properties in the model, the states are used to indicate the security situation of both attack side and defense side;the state transitions are used to indicate the dynamic security situation of the action for both attack side and defense side;the transition diagram is used to describe the transition processes of security status.Experimental results show that the model is more suitable for Network security modeling with simulating the intrusion processes of attackers and the security defense process.