Power system is a strongly nonlinear time-varying system, while it has large limitation when using traditional method such as Thevenin equivalent parameters to evaluate system voltage stability of nonlinear power system with dynamic feature. Based on dynamic analysis method of non-analytical complex power system, this paper puts forward normal impedance modulus margin index to access the N-1 system PQ node voltage stability, and uses the index to assess the node voltage stability weakest level of N-1 network. In addition, the minimum normal impedance margin value determines overall level of the system voltage stability, through opening important branches of the standard IEEE14 nodes system, it calculates the nodes minimum normal impedance modulus margin after branches opened. Then it compares this minimum normal impedance modulus margin values to find out the corresponding opened branch when the system voltage stability is weakest, and thinks this branch as key branch, so as to provide reference for power system planning and scheduling. The simulation results verify the correctness and effectiveness of this method. This work is supported by National Natural Science Foundation of China (No. 51577053).