The groundwater response and the variation in water inflow during extra-thick coal seam mining are of great significance for mine water disaster prevention and ecological environmental protection. The geological conditions of the 4^-2403 working face in Jianzhuang Coal Mine were taken as the engineering background. A physical similarity simulation was conducted to analyze the development law of the water-conducting fractured zone in the overburden during extra-thick coal seam mining. The variations in the groundwater level of the overlying Luohe Formation aquifer and the water inflow of the working face were investigated with the increase in the height of the water-conducting fractured zone. The results show that a threshold effect is present in the development height of the overburden water-conducting fractured zone. The development height is characterized by a “slow increase-sharp increase-stable” trend with face advancement. The sharp-increase point is located after the first roof break. A Logistic function is followed by the development height during face advancement. The water-conducting fractures are identified as the dominant factor controlling the mining-induced groundwater level variation. The onset of groundwater level decline is earlier than the time when the fractures connect with the aquifer. As the face continues to advance, water-level contours are distributed in an elliptical pattern along the advancing direction. A positive nonlinear relationship is observed between the water inflow and the development height of the water-conducting fractured zone during mining. A “slow-rapid-stable” trend is exhibited by the inflow with the increase in fracture-zone height. When the fracture-zone height no longer increases, the inflow is still increased slowly. The findings provide a reference for analyzing groundwater migration, predicting working-face inflow, and preventing mine water disasters under similar geological conditions.