Synchronous digital hierarchy(SDH)transmission network is widely used in the electrical power communication network

which has high security requirements because of its high survivability. According to the characteristics of the power communication network and SDH self-healing ring

the concept of topology units is proposed to split a network. The reliability of topology units can be calculated by analyzing its structure. And the fault severity index of topology units is obtained by considering the service and the node importance in the topology units. Finally

the risk assessment indices of topology units are calculated by means of the combination of reliability and fault severity index

then the indices for evaluating the overall risk of the network can be calculated based on the risk assessment indices of topology units. Simulation results show that the indices of the network risk and the risk balance of network based on the topology units succeed in measuring the various aspects of operational risk profile of the network and provide a useful reference for network optimization. Valuable applications are obtained both in the risk management and the control in the electric power communication network. This work is supported by Fundamental Research Funds for the Central Universities(No. 2014QN14).