A strong convection event on 21 July 2008 in Chongqing City was diagno sed. The developing mechanism of the strong convection weather was studied with the background of weather, the echoes of radar and the wind field computed by 4D VAR technique. The results indicate that the strong storm was caused by atmo spheric convergence, which was brought by upper cold air inrush and lower warmwet airflow intersection. Observation of radar shows that there were a weak echo fie ld and “adverse wind region". Low level wind field computed by 4DVAR shows that there was a convergence band before strong echoes. The convergence band is super posed with strong echoes in the middlelevel. And the upperlevel is divergen ce field. At the vertical section, there was an inflow at the rear of strong convection area. In the middle o f squall line there was a strong rising motion, which produced strong echoes. Th e strong rainfall in the front of weak echo produced the outflow, which converge d with warm and wet air and formed a thin outflow border line.