For on-line voltage stability monitoring requires high real time property, a set of online voltage stability monitoring system is designed in this paper. The power grid voltage stability is analyzed through the data from energy management system (EMS), to prevent power system voltage collapse through either the active preventive control or the user-defined preventive control according to the analysis results. This system is mainly developed with C++, Java language, with front-end Ajax framework combined which is unrelated to background. This system is friendly in man-machine interface, functionally comprehensive and easy to operate. It provides preventive control strategy in time for the operation personnel so as to ensure the gird reliability.