Abstract: Since impoundment of Majiagou reservoir in 2003, the seepage at the asphalt concrete core gravel dam had been serious. Anti-seepage treatments were carried out twice, but the leakage was still large. Inspection and data analysis showed that the core, the mat concrete gullet and the shallow bedrock made up the seepage passageway. The overall concrete cut-off wall deep into the bedrock was adopted in the anti-seepage treatment design, which reduced the seepage and enhanced the project safety, as well, expected effect was achieved.