Abstract: Based on the obvious differences in the watery of Taiyuan limestone aquifer under the floor of North China Mining Area, under the certain conditions, the aquifer could be a water isolation layer.In order to accurately determine the thickness of the water isolation layer under the floor and to predict the water inrush dangers from the floor, in combination with the actual conditions of No.21 seam floor in the first mining block of the Henan Xinyi Mine, the geophysical exploration, drilling explor ation and other means were applied to the survey and analysis on the watery of the limestone.The results showed that the floor limestone of the first mining block in the mine could be considered as a water isolation layer, thus the thickness of the floor water isolation layer could be increased to about 54 m in average. The water inrush C oefficient method was applied to evaluate and obtain the water inrush coefficient value of the major area in the first mining block all below the critical water inrush coeffi cient of 0.1 MPa/m in the normal section.Based on the previous floor grouting and reinforcement, a safety mining above the pressurized water could be realized.