Load margin is one of the most important index in static voltage stability, minimum load margin of nonlinear complex variable power system is defined. Power parameter is introduced using a dynamic equivalence method of nonlinear circuits, Lagrange function is constructed. It is proved the necessary condition of the maximum active power of nonlinear electric power network load node is that load static equivalent impedance mode is equal to dynamic equivalent impedance mode of load node, impedance modulus margin index is put forward which is used for assessing voltage stability. Under the same disturbance, the smaller the impedance module margin is, the weaker the voltage stability is, so the smaller the node load margin is. Therefore, the biggest load margin of the weakest node is calculated, it is minimum load margin. When impedance model margin of the weakest load node is zero, maximum active power is obtained and maximum load margin is calculated. Through simulation on IEEE30 nodes system under different disturbance ways considering load voltage static characteristic, it indicates that single node disturbance way and the increase of constant impedance and constant current load ratio improve minimum load margin of power system.