Abstract: In order to determine the safety upper limit of coal mining under the lake, to ensure that the cracks in overlying strata caused by mining do not affect the building and have no great influence on the production and life of the ground, to mine the coal under the lake as much as possible to reduce the loss of coal resources, based on the engineering background of 63209 mining face in a coal mine, we design and construct the maximum height of water flowing fractured zone. Considering the geological condition, the thickness of the protective layer of No.6 mining area in the western mining field is designed according to the requirements of the regulation, and the upper limit of Ⅲ lower coal seam under the lake is determined. The results show that the maximum development height of water flowing fractured zone in 63209 working face is 57.97 m. The bedrock weathered zone height of No.6 mining area in the western mining field is 11.5 m. The thickness of the designed protection layer is 13.2 m, and the final mining upper limit is -137.67 m.