In order to meet the needs of dispatching and monitoring the voltage stability of power grid in real-time, a monitoring system of voltage stability is designed based on Web. The programming of web in the server system is exploited by C++, Java and so on, while the interface is exploited by the script technique, and the database management system is exploited by SQL server 2000. The data source comes from the EMS(electic management system) text. During the development process using the tools of Myeclipse, according to the need of users, the developmented system can release the voltage stability data in the state of normal or N-1 like the load margin, area margin, the sensitivity of load margin and so on. The example shows that the system designed with the technique of Java and Extjs can improve development efficiency, better the program universality and satisfy the demands of real-time monitoring and analyse voltage stability.