Cascading failure is one of the main causes of blackout failure. Based on the OPA cascading failure model (ORNL-PSERC-Alaska, OPA), this paper analyzes the evolution mechanism of cascading failures and discusses the complex characteristics of blackouts. The OPA model is further improved by considering the hidden faults of relay protection and the complete derivation of failures chain. Multiple system-level risk assessment indicators are introduced to process the simulation results and the indicators can assess the risk of target system cascading failures at the macro level. the weak links of the target system are identified by counting the frequency of line faults. The related programs are written on MATLAB and the IEEE RTS96 system is taken as an example. The simulation results of cascading failure under different operating conditions are obtained, and the risk assessment indicators of the blackouts are calculated which prove the feasibility of this model.