This paper deals with two methods for computing the stability margin of static voltage,namely the optimalpower flow method and the continuation power flow method.In the investigation,first,based on the equivalencydefinition of the two methods for determining the critical points of voltage stability,the cause of the stability margindifference between the two methods,namely,the inconformity of power flow equations,is pointed out.Then,thecomplementarity constraints describing the reactive power and the terminal voltage of generators are introduced in anew optimal power flow model,and the increase direction of the active power outputs of generators,which is thesame as that of the continuation power flow model,is adopted.Computational results on the IEEE 9-bus system,the IEEE 39-bus system and a real provincial 748-bus power system illustrate that the stability margin of the staticvoltage and the bifurcation type obtained by the optimal power flow model accord well with those obtained by thecontinuation power flow model.