Abstract: 7608 working face of one mine is mining on confined water,and the limestone aquifer is only 30 meters away from the floor.In order to ensure safety mining and to prevent water inrush accident in working face, theoretical analysis and numerical simulation method is used to study floor failure depth in 7608 working face,and then the floor failure is measured with DC electric method.The result shows that actual floor failure depth is just 10 meters because of filling goaf.Measured result coincides with calculation result perfectly, so there is a water-resisting key strata with 20 meters in floor and 7608 working face has no water inrush accident danger.