In this paper the authors discuss the relationship between the pore fluid pressure and the fluid expulsion under the compaction, and suggests that the abnormal pore fluid pressure results from the increase of the capilary force due to the change of permeability and that a dynimic balance is maintained between them during the pregnancy of the fluid pressure. In sedimentary basins where deposition is continuous, when the abnormal pore fluid pressure exceeds the rock's shear strength, the capillary force will be decreased as a result of shearing fracturing of the rock, and the fluid will be expelled. Basins where intensed structural disturbance has occurred, when the decreased amplitude of load pressure reaches and exceeds the rock's tension strength because of a great amount of erosion, tension fissures will be produced in the rock and the fluid will be expelled.