As the priority of cyber-security arises world-wide, network security situation awareness (NSSA) and its application help to draw more attentions of researchers. NSSA is able to identify network activities, understand their intentions and evaluate the impact of these activities on the managed network, as well as to support an optimal security response to the security threats. It is a means of quantitative analysis for network security, with which network security management system can have a global view of security states of the managed network, find the intention of attackers, and make a management decision based on these findings. In the paper, the coverage of NSSA is discussed to redefine the concept of NSSA. Then a survey is given on the state-of-art of NSSA's research in the aspects of network security situation perception, comprehension and projection. Finally the features and challenges of network security situation awareness are summarized.