The traditional N-K fault analysis of power system generally adopts the method based on combination and traversal,which has the disadvantages of a large number of fault combinations and low computational efficiency.In this paper,the two-level programming model of this kind of problem is established,which can seek the fault combination causing the maximum amount of grid load through the way of optimization.Aiming at the fault combination determined by the upper-level problem,the lower-level problem simulates the operation of the grid after the accident,to meet the various types of power system operation constraints,by adjusting the generator output power to minimize the amount of grid load.Furtherly the flowchart of genetic algorithm to solve this problem is given.Finally,the validity of the model is verified through IEEE14 node and IEEE118 node example system.