Abstract: Take a to be built small (1) scale reservoir as the object of study. The reservoir is covered with grey-green thick conglomerate rock, and the surface of conglomerate rock is strongly weathered. In order to comprehensively evaluate the permeability of the dam foundation, 12 exploration holes was set along the axis of the dam foundation, and 65 sections of water pressure tests were performed to determine the depth of relatively impervious layer of dam site. The results of tests show that fissures are developed, water permeability is more than 5 Lu and the permeability is strong for the 11.0-16.2 m strong weathered conglomerate on dam foundation. The permeability of weak weathered conglomerate at the bottom is weak, it can be take as relative impervious layer of dam site. Meanwhile there are hidden risks around the dam and on dam foundation in the reservoir area, the anti-seepage should dig deep into relative impermeable layer by 1 m and anti-seepage should extend to both sides along the axis of the dam. The extended length is determined by the intersection of the normal water level line and the relative impermeable layer.