Abstract: Xiaonanhai reservoir dam is a rock-fill dam with inclined clay-core wall. The upstream thick inclined clay-core wall was filled with heavy silt loam. The embankment was behind the wall and a filter layer was set between. Diorite filled a large part of dam foundation and the original river bed located on the right. The thickness of dam foundation overburden was 8~12 m, of which the thickness of the strong permeable sand pebble layer was 3~5 m. Seepage prevention works at dam foundation were only 3 cutoff trenches of poor quality. The paper analysed the seepage of Xiaonanhai reservoir dam and pointed out the existing problems, worthy reference for rehabilitation and decision making.