A short-time heavy precipitation process occurred in Binhai New Area, Tianjin on the west coast of Bohai Bay in the morning of 4 September 2021, with the accumulated rainfall of 224.2 mm and the maximum hourly rainfall intensity of 110.9 mm[DK]·h-1, which both break the station’s historical records since its foundation. Based on the observations of intensive automatic meteorological observing stations, wind profile radars and Doppler radars, as well as ERA5 data, the paper analyzes the causes of the event, its relationship with sea and land environment, convective initiation, organized mechanism, transfer of water vapor and so forth. The results are as follows. (1) The convective cell, born on the sea surface, moves westward to land, strengthens after merging with backward newborn convective cell, and finally develops into a heavy precipitation system with meso-γ-scale rotation at the front end of a linear mesoscale convective system. (2) Convective cells are triggered at the intersection of two surface convergence lines. One south-north wind speed convergence line, located on the sea surface, is related to the smaller friction coefficient of water surface than land. The other east-west wind direction convergence line, located on the land, is formed by the residual northeasterly cold air and original southerly. (3) In the morning, the humidity on the sea surface is higher than that on the land, so the warm and wet air is carried from Bohai Bay to its west coast by the boundary layer’s onshore easterly flow, which is a favorable thermal condition for the occurrence and development of the convective system near the coastline. At the same time, the persistent warm shear line at 850 hPa provides dynamic support for the convective system’s linear organized arrangement. It should be noticed that the residual weak cold air triggers meso-scale frontogenesis at 925 hPa. (4) The secondary vertical circulation promotes the accumulation and upward transfer of surrounding water vapor, and the lapse rate of water vapor (vertical convergence of water vapor) has indicative significance for hourly rainfall intensity. The minute rainfall intensity is probably intrinsically connected with the meso-γ-scale rotation.