Abstract: To ensure safety of caving mining of special thick seam under the reservoir and aiming at the hydrogeological conditions of Daping Coal Mine, combined the upper and lower industrial network communication platform of mine, hydrological on-line automatic monitoring system for coal mining under reservoir consisted of reservoir water level monitoring system, Cretaceous aquifer water level monitoring system and underground working face water inflow monitoring system is set up. Based on Windows98/2000/XP environment, the automation from the data acquisition and processing to hydrological information release is achieved. By running and applying during N1S2 face mining period, monitoring results show that the floor water-resisting layer in lower aquifer segment of the Cretaceous aquifer water floor is failure and water conductivity, the upper weathered zone aquifer segment has no hydraulic connection with face gushing, coal mining under reservoir is safe and reliable.