A mesoscale numerical mode named Weather Research and Forecasting (WRF) was utilized to simulate a squall line occurring on 10 June 2006 which severely impacted Zhejiang Province by using conventional observational data, mesonet data, and cloud motion wind data. The results showed that the mesoscale convergence line/zone in the boundary layer was associated with organized strong convection. There was a simulated mesoscale convergence line in eastern Ningbo besides that a gust front formed by forward outflow and lay in front of the squall line. The local convergence line in eastern Ningbo, which prevented the eastward propagation of strong instability at the top of the boundary layer, could be characterized by a weak stable stratification and that it is drier in the west and wetter in the east. In addition, a mesoscale closed vertical circulation with about 10－20-km scale generated near the surface to the east of the local convergence line. In reverse, the vertical circulation prompted weak easterly flow to enhance the local convergence line. It produced a new gust front when the existing gust front and the local convergence line met, which could intensify humidity at the top of the boundary layer and release strong instability gradually. Especially in the front of the new gust front, it resulted in intensification of upward movement and breaking out of strong convection. 