The ORNL-PSerc-Alaska (OPA) model is a blackout model proposed by researchers at Oak Ridge National Laboratory (ORNL), Power System Engineering Research Center of Wisconsin University (PSerc), and Alaska University (Alaska). Although the OPA model is a landmark study, it has two limitations. First, there is a significant difference between simulation and practice in transmission line outage and update; and second, the simulation of cascading failure and the probability distribution of blackout size are in general not accurate enough. Hence, an improved OPA model is proposed in this paper to address these limitations. The proposed model contains two layers of iteration. The inner iteration describes the fast dynamics of the system and considers the influence of power flow, dispatching, automation, relay protection, and so on. The outer iteration describes the slow overall system evolution and is concerned with the update of the power grid, operation modes and planning. Such a model can be applied to practical large-scale systems. Furthermore, based on the Value at Risk (VaR) and Conditional Value at Risk (CVaR), two new complementary blackout risk indices are defined, which reveal critical characteristics of blackouts and are used to evaluate security levels of power systems. The effectiveness of the improved OPA model is verified by the simulations concerning the Northeast Power Grid of China.