Through detailed analysis of the defects of OPA and its improved model， a new model based on the risk theory is proposed. The new model includes internal and external circulations. The internal circulation is based on traditional linear programming of direct current power flow to describe the cascading line overload and failure. The external circulation includes load growing and line capacity improvement， and the later is based on the risk theory. Whether to improve the line capacity and the degree of line capacity improvement are determined by risk index of line capacity improvement， delay time of capacity improvement， improvement degree function and line risk value. A new prevention strategy for cascading failure is proposed by identifying the high-risk lines and defining optimal risk index of line capacity improvement which minimizes capacity improvement cost and load loss. Finally， by comparing the outage probability， load loss and risk index between the original model and the new model， the reliability and practicability of the new model are illustrated.