Using the data obtained by conventional observations, ground densified automatic weather stations, Doppler radar observations and mesoscale TJWRF model， we analyze the heavy rain occurring on 6 July 2009 at Ninghe District of Tianjin and highlight the trigger mechanism of coastal local storm caused by sea breeze front of the Bohai Gulf. The results show that the heavy rain occurred in the favorable weather conditions, and there were local instability and adequate vapor conditions. Sea breeze front itself has a convergent uplift area where weak convection exists; when sea breeze front moves to the unstable and adequate vapor areas, it will strengthen the convergence positive movement of convergent uplift area and lead to the new storm development. The meeting of the sea breeze front and thunderstorm will play a strengthened role in its development. By analyzing the data obtained by mesoscale WRF model, we can see that at the warm and dry ground in front area near the north end of the wet air interchange, the intersection of the two convergence lines is easy to stimulate strong thunderstorms, and thunderstorm accrues at the intersection of the north of the subaerial warm and dry front zone and humid air. Sea breeze front plays an important role in the development of the thunderstorm.