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An exceptional shorttime strong rainfall happened in the Beichen district of Tianjin at the midnight of 8 Aug 2017 and its intensity surpassed 70 mm·h-1. Using the methods of multidata superposition analysis, quantitative analysis of radar echo parameters, and lowlevel wind field retrieval, etc., RMAPS (Rapidrefresh Multiscale Analysis and Prediction System) products and various observation data are used to analyze this rainfall. The results indicate: (1) The multicell storms in the southern extremity of MβCS (Mesoβscale Convective System) during the backpropagation caused this rainfall. The merging process had an important impact on the structure and enhancement of the storm system. After merging, echo volume parameter 〖WTBX〗V〖WTBZ〗40 〖KG*3〗(the volume of the reflectivity above 40 dBz) increased by 23.3%. Before the rise of 〖WTBX〗V〖WTBZ〗40, variable parameter 〖WTBX〗F〖WTBZ〗V40-up6 (the change of strong echo volume of above 40 dBz above 6 km height in 6 min) suddenly dropped, indicating that the echo was about to develop explosively. (2) The ground convergence line formed by the cold pool outflow and surface southeast winds was the key to maintain the convective system; the low level wind shear also influenced the trigger and development of convection; accompanied by heavy rainfall, CAPE suddenly dropped from the peak and the low level vertical wind shear index α increased, indicating the relationship among unstable energy, vertical wind shear, and convection process. (3) The large value area of the instability parameter SWEAT had an early indication of the heavy rainfall area; the lead time was about 3 hours; and the location where the surface warmwet area from the Bohai Sea and pseudo equivalent temperature dense zone met was the focus of falling area nowcast.