The thunderstorm gale as powerful as grade (10-13) overturned a fishing ship along the Shandong Coast on 29 April 2021. The causes of this thunderstorm gale process are investigated in this paper based on the data of Doppler weather radar and densely-distributed automatic weather station, ERA5 reanalysis data and the conventional observation data. The results show that the cold and dry air coming from northwest behind the northeast China cold vortex superimposed the lower-level warm ridge, and the strong potential instability stratification and vertical wind shear provided favorable environmental conditions for this severe convection weather. Under the influence of the strong upper-level northwest steering currents, the advection of both the Qingdao thunderstorm and the gust front were fast, and the angular momentum of downdraft was increased due to momentum downward-transporting. The gust front lasted for a long time and moved fast, directly causing thunderstorm gale of grade 10-13 to hit the places it passed through. Dry air in the middle-low troposphere and evaporative cooling of upper-level hydrometeor during its falling process, formed a cold pool with thickness up to 120 hPa near the ground. Allobaric wind formed between the pressurization area at the rear of the large-scale cyclone superimposed with the small-scale cold pool and the decompression area of the cyclone. Density currents between the cold pool and warm ridge in Rizhao superimposed with allobaric wind led to enhancement of surface wind. Considering the orientation of the pressure gradient changed after the high pressure cold-core entering sea. Wind direction became rotated so that the risk of fishing boat overturning increased. The evaporation, gradient of the frontal zone, intensity of density flow and superposition effect of the multi-scale weather system over Rizhao, were all stronger than those in Qingdao, so that the thunderstorm gales above grade 10 in Rizhao City, which lay on the edge of Qingdao thunderstorm, had a greater power and scope than those in Qingdao City, that was affected by the main body of the thunderstorm.