Protection failures have serious threat to power grid security. It is a typical dynamic equilibrium characteristic when line parameters are revised to mitigate cascading failures after the fault occurs. Based on complex network theory a method of risk assessment, considering protection failures and power grid dynamic equilibrium characteristics, is proposed. Firstly, this paper defines a concept of protection failure severity, measuring the impact of protection failure on power grid security, based on hidden failures. Secondly, a variable weighted model, combined with protection failure severity, is established to characterize the dynamic of power grid. Then, risk theory is introduced to assess power grid security from the point of complex network theory. Finally, the New England 39-bus system is taken as an example and the validity of proposed method is verified.