The permeability characteristics of rock mass discontinuities are important in the stability of hydropower station projects. We propose a large-scale in situ seepage testing method and use this method to test gently dipping bedding faults (C3 zone) and steep faults (F14) in a hydropower station construction field in China. The in situ test results are compared with those of both undisturbed and reconstituted specimens. The comparison indicates that the largest critical hydraulic gradient and the smallest seepage permeability coefficient are obtained via in situ tests because they are performed under stress states that simulate the natural stress of the surrounding rock mass. The natural stress of the surrounding rock mass cannot be reflected in tests of undisturbed and reconstituted specimens.