Overload of transmission lines is considered as one of the most important factors which results in further deterioration of the cascading failures in power systems. Therefore， based on the failure probability model for transmission lines， the risk indices of cascading failures which reflect the operating state of the system is calculated. A recursive algorithm is used to build the cascading failure tree which can record every state on cascading failure path and its relevant risk indices. Based on the operational reliability evaluation framework， combining with dynamic power flow calculation， a cascading failure operational reliability evaluation is carried for IEEE RTS test system. The risk indices of cascading failures under different transmission line rating capacities and load levels are calculated respectively. The calculation results illustrate that the indices can reflect the real system state and provide operators with early warning information which points out the weakness of the power system.