To assess the security of communication networks for integrated substation automation systems, a systematic approach based on index system is presented. An analysis about the actualities and security risks of the communication network was performed firstly. Then, the index system of security assessment was established according to assessment standards about information security. At the same time, index weights were assigned and formulas to valuate index were proposed. Finally, this assessment approach was used to assess security levels of a real communication network, and its effectiveness was validated. Therefore, a guidance and assessment can be provided to communication networks between the control center and substations through the approach in this paper.