Based on conventional observation data, Doppler weather radar data and ERA5 reanalysis data, the physical process responsible for the enhancement and maintenance of a strong squall line affecting the Bohai Sea Area during the night of 31 July 2021 is analyzed. The results show that coastal terrain and sea surface temperature were the key factors for the intensity of squall lines. The warm water areas in the central and western Bohai Sea had favourable conditions for thermodynamic instability, and the areas with high wind speeds corresponded to the areas with high sea surface temperatures. The thermodynamic instability around Bohai Bay was the strongest, and the horn-shaped coastal terrain around Bohai Bay exhibited “the effect of funnelling”, resulting in significant wind field convergence on the windward side of the coast. This was conducive to triggering coastal convection. In the early stages, the cold pool outflow formed by multi-cell storms on land converged with environmental airflow, causing the storms to propagate eastward and move into the Bohai Bay. The uneven onshore wind caused by the complex coastal topography in the northern Bohai Bay was helpful for the rebirth of scattered thunderstorm cells along the coast, and the repeated incorporation of scattered thunderstorm cell made the onshore multi-cell storm continue to strengthen, forming a squall line.The herringbone squall line formed after entering the sea, accompanied by the merging of cold pools, and at the same time, a mesoscale vortex strengthened the squall line near the northern coastline of Bohai Bay. In the process of squall line strengthening at sea, the boundary layer frontogenesis associated with the warm water area and the cold pool played a critical role, finally making the squall line develop and strengthen in the warm water area of Bohai Sea but weaken and dissipate in the cold water area.