Sandstone is encountered in many hydropower projects in China, and its permeability evolution characteristics during the deformation process are one of the most important issues in engineering design. In this study, microstructure observation and mineral content analysis of a sandstone are performed, and triaxial flow experiments under different fluid pressures are carried out. The experiment results show that the permeability evolution with volumetric strain is associated with microcrack development in the process of rock deformation and failure. The volumetric strain is closely related to the permeability and fluid pressure, and it plays a role of linking the flow field and the stress field. A conceptual model is suggested to describe the permeability evolution of the rock. Scanning electron microscope analysis results on some failed specimen pieces illustrate that the fluid pressure plays a role in the development of microcracks. A high fluid pressure can accelerate crack propagation and rock f...