Abstract: Structures, organization of particles, permeability and the hydraulic gradient of the faults in the foundation of nanwan reservoir is important to the safety of nanwan dam. The hydraulic gradient gained from the permeability testing in the site is higher and can not reflect the real hydraulic gradient of the heavily fractured rock. The analysis of engineering geological and hydrological conditions about the foundation of nanwan reservoir is carried out and the computation of the seepage is put forward. The seepage field of the dam is gained from the computation and the datum of long-term observation.The result indicates that the dam is safety.