This paper proposes an advanced risk assessment model of cascading failures. Taking the characteristics of the line failure probability and the impact of real-time power flow into account, the model uses the method of the line segment fitting to calculate the probability of line outage. By analyzing the double impact of cascading failures both on the supply side and the electricity users, the paper uses the bus low voltage, the power flow relative shift amount and the relative amount of load loss to assess the consequences of the cascading failure. The proposed algorithm recognizes the relative transmission section by the active power increase factor, and then selects the next branch failure link contained in the transmission section to avoid the whole network traversal search. Finally, the simulation of IEEE 39 bus system proves the validity of this method.