Abstract: The stability of the joint between the groundwater reservoirs in the mined-out area of the mine is one of the key factors for the safety and stability of the entire underground reservoir. To investigate the deformation and failure law of the joint under the working conditions, laboratory simulation and FLAC^3D numerical simulation technology were used to analyze and monitor the force and deformation of the joint under the working conditions. The experimental and simulation results show that the inside of the boundary between the dam and the joint is the part with the most severe pore water pressure; the softening effect of water reduces the mechanical properties of the contact surface, and the moisture immersed in the pore crack makes the specimen fuller, which indirectly enhances the ability of the dam to resist external pressure, and enhances the mechanical strength of about 23%; the ultimate instability and failure of the joint is the consequence of the gradual expansion and extension of the internal damage at the junction, so we choose the joint material and pay attention to the connection. Reducing the pore cracks inside the joint is a crucial step to ensure the safety and stability of the dam.