To improve the accuracy and sensitivity of the security evaluation of wireless sensor networks (WSNs), we introduce the concept of security situational awareness. We adopt the theory of set pair analysis to evaluate the security situation of WSNs, and the value of security situation is then applied to assess the threats this network has encountered. In the experiment, we use the KDD Cup 1999 data set to simulate wireless network running situation, and analyze the changes in the value of security situation in 11 network attacks which are simulated by changing the quantity of attacked nodes in the network under different intensities. Experimental results prove that the proposed model can improve the accuracy of network security assessment, and divide the network security situation into three levels. It is more sensitive than the security entropy based models in detecting medium and low intensity attacks.