Abstract: It is introduced that the principle and stress measurement data achieved by hydraulic pressure fracturing method.This paper discusses the mechanisms of impermeable layer water-resisting in coal seam floor and the confined water rising along the fractures,as well as the relationship of in-situ rock stress,confined water pressure and fracture extension.The in situ rock stress prevents the rise of confined water and water inrush for thick floor aquifuge,but for thin floor aquifuge the in situ rock stress would break coal seam floor,resulting in confined water inrush.