Based on conventional automatic meteorological data, FY2G satellite data, Doppler radar data and other related data, a localized severe storm under the influence of subtropical high in Chongqing is analyzed and simulated. This paper discusses evolution process of mesoscale convective system (MCS), triggering and maintaining mechanism. The results show that mesoscale environment of high temperature, high humidity air mass and extreme unstable atmospheric stratification conditions were provided under the influence of subtropical high. The triggering key factor of the local convective storm was mesoscale convergence line in shallow boundary near the ground. The convergence line was generated by combined contribution from thunderstorm high pressure of MCS in the central and western Sichuan and Chongqing Basin and the surface thermal low in Chongqing. Small range of cold pool outflow was generated by convective storm triggered by convergence line. A new convergence line was formed by small range of pool generated by convective storm triggered by mesoscale convergence line and ground ambient wind, strengthening development of convection storm and triggering new convection cells. The convective cells moved slowly northward under the average south wind in the bearing layer. Thus the cold pool outflow and the boundary convergence lines caused convective cells to propagate northwestward and maintain a long time.