An improved OPA model is proposed. This model contains two layers of iteration. The inner iteration which describes the fast dynamic of the system considers the influence of the power flow， the dispatching， the automation， and the relay protection， etc， while the outer one which describes the long term slow dynamics of the system considers the impact from power grid upgrade， network operation， planning and other factors. Furthermore， based on the VaR and CVaR， a method which could be used to evaluate the blackout risk of the power system quantitatively is presented. Appropriate preventive control measures are proposed as well. The effectiveness of the improved OPA model is verified by the simulation results concerning the northeast power grid of China.