Abstract: In order to obtain the water inrush mechanism of roof under ultra thick and low permeability aquifer, the variety of methods were carried, including obser vation of drilling fluid leakage volume, geophysical prospecting, observation of television and numerical simulation.The results showed that the presence of low permea bility thick- sandstone aquifer underlying impermeable shale layer in the overburden rock and the ratio 12. 51 of the height of the fractured zone to the mining height wh ich spreaded to the sandstone aquifer provide water source, channels and other internal conditions. Through the study of mine transient electromagnetic method and nu merical simulation, the storage water space which under mining conditions the combination structure of sandstone and shale ( hard rock and soft rock) lead to disturban ce shale remodeling to form was determined, when the water closed in the storage water space reached a certain degree, with the periedic weighting of main roof and a large area of collapse, the separation water bag burst and instantaneously spurtted into mine, leading to water irruption. On this basis, summing up the methods and ex perience of abscission layer water effective drainage in process of mining, which provided a valuable reference for similar to water damage prevention.