In order to ensure the safety of later-stage mining in a copper mine in Yunnan, a combined method of rock mechanics testing and numerical simulation was adopted to investigate the influence and failure patterns of stepwise mining in the goaf under large-scale water body. The rock mechanical parameters required for numerical simulation were then obtained by combining the field investigation and discounting the indoor rock mechanical parameters using the Hoek-Brown criterion. The mined-out ore body between 1 367~1 245 m, as well as the unmined ore bodies between 1 245~1 200 m and 1 367~1 409 m, were simulated using the finite element software ANSYS. The results indicate that: The main factors threatening the safety of the mining area are the tensile stresses in the surrounding rocks above and below the goaf and the stress concentration in the corners of the mined-out areas in the lower and upper parts of the goaf. Extending the goaf downwards does not significantly affect its safety, but when mining was carried out upward from the 1 394 m level to the 1 409 m level ,the safety of the goaf deteriorates significantly. Considering the current condition of the mining area, it is recommended that upward mining be discontinued and the goaf be filled to prevent major roof collapse in the goaf, thereby avoiding safety accidents involving water inflow from the nearby water bodies.