Network security situation awareness is vital important for network security supervision. In order to obtain the network security situation effectively, a multidimensional assessment method is proposed in this paper. The method is composed of three dimensions at different levels, namely vulnerability, threat and basic operation, with quantitative calculation method for each index. In the service layer, CVSS standard is adopted to assess the vulnerability situation, and simplified DREAD model is chosen for the threat situation. In the node layer, the vulnerability situation in the service layer is added with a weight, the threat situation in the service layer is accumulated according to attack paths based on Markov model, and the basic operation situation is evaluated by D-S evidence fusion of several host and network performance index. In the network layer, each situation equals to weighted summation of corresponding situation in the node layer. Experimental results show the ease of use of this method, and multi-dimensional situation depicts the overall safety evolution process of network system accurately and intuitively.