Abstract: The permeability is a very important factor to determine the hydraulic characteristics of rock mass. To study the permeability of deep rock mass, high-pressure borehole water injection tests are carried out to analyze the four segments of rock masses in Dongtan Coal Mine, and numerous data are obtained. The test results and analyses of data show that the rock mass goes through the process of “water resistance-seepage guiding-stable seepage”. The permeability of rock mass increases with the test process. The thick mudstone under the tunnel of Dongtan Coal Mine exhibits the behavior of low water anti-pressure and permeability, while the thick sandstone and other two rock masses show that of high water anti-pressure and low permeability. The four test rock masses are of low permeability. The regression results indicate the exponential relationship between injection pressure and flow rate with high regression coefficients. The curves of injection pressure and flow rate of thick mudstone and interbed exhibit significant segmentation expect those of thick sandstone. The relationship between injection pressure and equivalent fracture width shows obvious exponential relationship as well, and two significant segmentations appear, that is, stability before mutation and sudden increase after mutation. Whether seepage in the rock mass happens or not can be judged by the relation between and 0. The conclusions will help further understand the permeability of deep rock masses under high water pressure and may make an important reference for safety mining of deep coal mines.