Abstract: The observational data from Doppler radar, SAFIR3000 three-dimensional lightning position system, microwave radiameter, and from high-density meteorology automatic stations were used to analyze a big hail case caused by a multicell hailstorm which connected with a sea breeze front on the west coast of the Bohai bay. The characteristic of merger process, lightning activity parameters and quantificational structures of hailstorm were studied. The results show that: Firstly, the humid and cold sea breeze convergence line in the local instable region on ground was the mechanism for storms’ triggering and developing with the time of early warning for hailstorms of 2 h. Secondly, there were two merger types of γ mesoscale convective cells and small β mesoscale convective cells, that is, the inde pendence type, and the feeder type. As cells merging, the cloud bridge between two cells emerged at the height of 4-6 km or 9-10 km. Thirdly, at the time of merging, the development of storm was limited and hailfall and the peak of lightning emerged after merger. Fourthly, the relationship of the evolution of quantificational storm 3D structure (time height cross sections of the radar parameters of Zmax,Zmean15 , V 40 , V 50 etc.) with lightning activity was investigated, with the result that the radar parameters of V40Fup, V40 Fdown, SET11 were able to describe properly the feature of merger process and the change of hailstorm structures, and pulled ahead of the total lightning flash peak for 12 min.