Risk theory is applied in static security analysis of power system, and the risks mainly include line overload risk and voltage overlimit risk. For these two kinds of risks, several main factors related to overload and voltage are considered, and the risk assessment models of overload and voltage overlimit are established respectively based on an appropriate consequence severity function. Finally, the simulation calculation is completed in the New England 39-bus system, and the importance of transmission lines is evaluated and sequenced according to these two risk indicators.