Beijing electric power grid locates at the key point of Beijing-Tianjin-Tangshan electric power grid， and takes high proportion of energy from neighboring electrical grids. So on-line monitoring and analysis of voltage stability become more and more urgent. In this paper， an on-line voltage security assessment system is built based on the DSA PowerTools to assess the voltage security of Beijing electric power grid on line. It has many functions of voltage security assessment， such as assessing voltage security， screening contingencies， identifying preventive measures and off-line analyzing stability. A data-exchanging procedure and Web browsing to system analyzing results are developed for the application of Beijing electric power grid. It can be seen from the operation of this system in Beijing electric power grid that the whole system works well and can fulfill all functions of voltage security assessment. Moreover， the contingencies setting by the user-interface and edition of input data file can be realized through data-exchanging procedure， and the analysis results can also be browsed through Web page.