In the proposed risk assessment model， the failure possibility of overhead lines is regarded as a random fuzzy variable. Based on the evidence theory， the failure possibility model for overhead lines is constructed which can better reflect the effect of the surroundings on the failure possibility. In the risk assessment model， the utility function is used to measure the degree of dissatisfaction induced by failures and the proposed risk indices can sensitively reflect the trends of risk indices and accord with the actual power system operation. Finally， the Beijing power grid is used as a research example to show the feasibility and effectiveness of the method proposed.