The existing power grid security risk assessment methods do not fully consider the impact of different primary equipment, load importance and secondary system performance. Grid static security risk assessment method considering the influence of secondary system based on the improved severity model is proposed. Firstly, the effects of hidden faults of relay protection, automatic reclosing success rate and automatic load-cutting devices on risk assessment are analyzed, and corresponding risk assessment model is established. Then, by quantifying the importance of lines, buses and loads, a risk index quantification method of the improved severity model is proposed which introduces the importance degree factor into severity model. The proposed method is applied to the improved IEEE-RTS79 testing system. It is verified that the proposed model and method can effectively identify the weak links in the power system, and are more in line with the system reality than traditional risk assessment methods. The proposed method can provide a reference for power system operation, maintenance, and risk management.