Network security is an important guarantee to reliable work of information system. This paper proposes a guideline system for the network security situation analysis according to the demand of the online evaluation analysis of the security situation of the electric power information system. Facing to electric power information system, we mainly discuss the quantitative methods for attacking situation and the severity correction method for the attacked equipment assets. Combining the actual situation with self characters in the electric power service systems and networks, we define a security situation evaluating framework, propose an evaluating algorithm, and supply a security situation online-assessing system framework for electric power information system.