Abstract: Based on Doppler weather radar products and conventional and intensive observational data, important impacts of complicated sea breeze fronts over Shandong Peninsula on the breaking and reorganizing process of a long-lived squall line on 30 June 2016 as well as the formation of extreme gale and hail disaster are investigated. The results indicated that: (1) The initial weak convective cells were triggered in the cold front convergence area. When they intersected with horizontal convective rolls as the convective system moved towards more unstable area, the convection developed rapidly. The developing convective cells were gradually organized into a linear squall line with an east-west direction. (2) The squall line broke off during its moving in the plain, which was related to the squall line gust front and two sea breeze fronts formed between the Yellow River Delta and the Bohai Bay with different moving directions. The two sea breeze fronts intersected with the middle section of the gust front, which induced new convective cells and caused the convective system to propagate more rapidly. Finally, the squall line system broke. At the same time, the western section of the broken squall line weakened gradually since the warm and wet inflow in the lower levels was cut off. (3) The newly born cell developed to the northeast, which was triggered by the outflow of the west residual storm and separated from it; meanwhile some other convective cells formed in the tail of the eastern section of the original storm (propagated to the southwest). The re-organization process finally led to the formation of a new squall line with typical bow characteristics, larger horizontal scale, and approximately northeast-southwest direction. (4) The extreme gale and the maximum hailstorm during the storm activity occurred at the "bow" of the squall line in the early stage of re-organization of the squall line. The extreme gale on the ground was a result of the comprehensive effects of cold pool density current, rear inflow jet and front downdraft, which was completely separated from the rear inflow jet and corresponded with water mass falling, and the falling water mass plays an important role during the extreme gale. (5) The Yellow Sea breeze front on the east side of Shandong Peninsula encountered the squall line moving from northwest to southeast, which strengthened the uplift, the water vapor supply and the organization degree of the front side of the storm, and provided favorable conditions for long-term maintenance of the squall line.