Combined with the electrical characteristics of power network, a method has thus been proposed based on complex network theory and network maximum flow algorithm to locate the key lines of power network. Firstly, by calculating the power network admittance matrix, the connecting lines are assigned with corresponding weights. Then, based on the maximum flow algorithm of the network, the maximum flow of the connecting line can be worked out, with a synthesis of the admittance and maximum flow of the line as new state indicators for identifying key lines. Finally, the simulation analysis has been carried out of the 39-node system and 118-node system of the IEEE. The results show that the system has strong robustness to random attacks, with the efficiency of the system almost unaffected by the random selection and removal of components, while the efficiency of the network decreases sharply with deliberate attacks on lines with high centrality index, with a decrease by about 60% the efficiency of the IEEE 39 system and a decrease by about 40% the efficiency of the IEEE 118 system.