Abstract: This paper constructs a network security situation assessment model on the basis of biologic Immune theories. The concepts of self, nonself, antibody and antigen are defined, and the immune mechanisms of self-tolerance、the evolution of antibody cell、dynamic immunologic memory、immune surveillance are presented. With the relationship between the antibody concentration and the degree of illness in the biologic immune system, a real-time model of situation evaluation for network security based on artificial immune system is proposed. Key words: artificial immune system; network security situation assessment; network