Abstract: The filling materials of the slag dam in hydropower project mainly consist of weathered sandstone from previous excavation of the buildings. The dam with an inclined wall adopts the composite geomembrane to control seepage, while the dam foundation uses curtain. The composite geomembranes are respectively inserted into the connecting plate, wave wall and left guide walls of spillway around it, forming up a close seepage control system. Judging from the operation of the water storage after completion of the project, the seepage quantity of the dam is only slightly larger, and the sound seepage control status is ensured.