In order to study the failure height of overlying strata after mining in the lower layer working face under thick alluvium and thin bedrock in Zhaogu No.1 Mine, theoretical analysis, numerical simulation, borehole flushing fluid leakage observation and borehole peeping method were used. The research shows that the development height of falling zone is 6.93 ~ 27.72 m and the development height of water-conducting fracture zone is 22.17 ~ 71.73 m after mining of lower layer working face calculated by empirical formula. The numerical simulation results show that when the mining height of the lower layer is 2.5m and the cumulative mining height is 6m, the maximum ratio of caving to mining is 2.61. When the cumulative mining thickness is less than 6m and the bedrock thickness is not less than 30m, and the clay layer thickness is greater than 5m, it can be considered that the clay layer thickness can block the upward development of the water-conducting fracture zone. Using the field measurement method, the falling zone is 15.26 m, the ratio of caving to mining is 2.62, the water-conducting fracture zone is 55.59 m, and the fracture ratio is 9.58 in the cumulative mining thickness is 5.8 m. The law of the failure height of the overlying strata after the mining of the lower layer working face can provide a theoretical basis for determining the thickness of safety coal ( rock ) pillar of waterproof and sand control in the lower layer working face.