Abstract: In order to explore the failure of the upper rock layer caused by the mining of the underground coal seams in the thick loose layer of the mountain area and research the impact of the safety of coal seams under the water body on the above reservoir and dam body, we take the 9-218 working face of Fangshan Dianping Coal Mine as the research object and analyze the impact of the reservoir and dam after mining by the method of empirical formula, probability integral method, and numerical simulation method.The height of the crack zone obtained by the theoretical algorithm and numerical simulation method is 46m, and the height of the maximum protection of the coal and rock pillar is 54.4 m.The thickness of the base rock near Zhangjiaer Dam is 86～367 m.The thickness of the base rock is greater than the maximum protection of coal and rock pillar height, thus coal mining has little effect on the safety of the water of Zhangjiaer Reservoir.In the analysis of the impact of the dam body, the calculation result of numerical simulation result is greater than the calculation result of the probability points method, which is due to the complexity of the mountainous areas, the expected results will be affected, and the simulation results consider the terrain factors, and consider it according to the dangerous working conditions.The maximum value of stretching and deformation is 7 mm/m, which exceeds the maximum deformation specified in the ”Code for Coal Pillar Retention and Coal Mining in Buildings, Water Bodies, Railways, and Main Tunnels”.The coal mining has a certain impact on the Zhangjiaer dam.