For solving the problem of slow calculation and low accuracy in the existing methods, a method that can get the operation mode combination of power system by genetic algorithm is proposed. Firstly the operation mode is mapped to nodal impedance matrix of power system. The fault current and branch parameters are expressed by the element of nodal impedance matrix. Then the mathematical model is established for getting the optimal combination of power system with the objective of smallest and biggest fault current and branch coefficient respectively. Finally, considering the feature of model which is discrete and nonlinear, the model is solved by genetic algorithm which can get the combination of power system in extreme case quickly and accurately. The correctness and practicability of the proposed method are verified by an example.