In the networks, Situation Awareness (SA) is a new concerned issue for the security. Usually, SA implementation includes three stages. They are the knowledge of SA, the situation estimation of SA and the situation prediction of SA. Among these stages, the situation prediction is the most difficult and treasured. Prior predicting, the situation estimation has to be established first. The major concerns in this work are to propose the methods to establish the model to estimate SA, to apply Dempster–Shafer theory to estimate SA. Finally, based on the knowledge and databases from the experts and experiences, the performance evaluation is proposed.