Abstract: By introducing the fracture mechanics into fluid-solid coupling analysis, the mechanism of seepage-fracture coupling is established. Baesd on the existing computation modules of FLAC^3D, the programs of seepage-fracture coupling in fractured rock mass are developed by use of the FISH language. The coupling mechanism of the model lies in that the hydraulic gradient acts on the stress units as seepage volume strength, the seepage pressure acts on the opening part of cracks as surface force and induces fracture and growth of discontinuous cracks, and the crack growth leads to the increase of permeability coefficient and ultimately to the change of seepage field. The theory of seepage-fracture coupling is applied to unlined pressure tunnels. The laws of hydraulic fracture, distribution of seepage field and leakage ratio of the inner are studied. The results are obtained as follows: (1) from the tunnel border, the rock around the unlined pressure tunnels stretches outwards and three zones are formed, that is, tensile-shear splitting zone, compressive-shear splitting zone and no disturbance zone; (2) radial displacement occurs at the tunnel border during the leakage of inner water; (3) the leakage ratio first increases, then decreases and finally keeps steady. The hydraulic splitting factor of pressure tunnels under inclined ground is proposed and closely correlated with the geometric and mechanical characteristics of cracks and fracture toughness of the rock. For the design standards of hydraulic tunnels, it is suggested to establish the safety control standards matched with the hydraulic splitting factor. The mechanism of seepage-fracture coupling provides a theoretical basis for the design of unlined pressure tunnels in China.