A sudden rainstorm process happened in Tianjin on 4 September 2015. The analysis based on various observation data showed that the southerly flow met with the northeast wind at the bottom of the high pressure, forming the convergence in the mountain front and triggering thunderstorm. After the thunderstorm was formed, the “backward propagation” mechanism formed, matching up the southerly wind in the midlow level, and forming the southnorth “train effect” which led to the happening of heavy rainfall. The precipitation caused by upper trough developed into the cool pool near the surface layer. The divergence flow met the easterly flow, triggering the new thunderstorm in the downstream of precipitation area. This “forward propagation mechanism” made the rain belt move faster than that of upper trough. And when the divergence flow met with the easterly flow, smallscale convergence cyclonic circulation formed, making the precipitation increase in front of the convergence line. After the uppertrough precipitation moved across the urban area, due to the mesoγ scale convergence effect, which triggered the formation of mesoγ scale convective cloud, the heavy precipitation once again appeared in the urban area of Tianjin City. During the shortterm forecasting, besides the 〖JP2〗analysis of largescale environmental background,〖JP〗 forecasters only considered the precipitation field from mesoscale model, ignoring the analysis of mesoscale environmental field. The results showed that although there is deviation between the precipitation from model and observed precipitation, the prediction of the mesoscale environmental field could provide reference for the prediction of sudden heavy rainfall in the short period of time (24 h).