Reservoir water or sewage leakages often occur beneath buildings of man-made reservoirs or mine tailings reservoirs in karst areas because of the complicated geological conditions. To clarify passages of these leakages, geophysical prospecting tools are commonly deployed on in dams, abutments and other lands of reservoir areas, which cannot detect and evaluate the karst collapses at the bottom of the reservoirs directly, leading to poor effect of control to this problem. This paper takes the karst leakage passage beside culvert of the first-class hydropower station on the Jinjihe reservoir in North Guangxi as an example to study the principle, field working method and effect of detecting karst leakage passages in reservoir areas using the self-potential method on the water. Outcropping formation in the study area mainly consists by of limestone intercalated with dolomite, dolomitic limestone of Devonian Liujiang Formation (D3l) and dolomite, limestone, argillaceous limestone of the Devonian Donggangling order (D2d), where developed intermedium-degree hidden karst. When the outlet pipe of the hydropower station in the area releases water, a strong negative center regional field of self-potential on the water can be producd. By dividing the regional field of self-potential on the water using the circumferential mean method and calculating the residual local anomalies, 6 secondary negative center residual local anomaly areas of self-potential on the water covered by a strong regional field, which are quasi-equiaxis or elliptical shape, are identified. Checking in the reservoir by the diver, these 6 negative center residual local anomaly zones correspond to karst collapses, karst opening or leaking skylight at the bottom of the reservoir, respectively. Among them, the negative center residual local anomaly with quasi equiaxis shape near the survey points 46-48 of line 16 corresponds to the largest karst collapse at the bottom of the reservoir. The diameter of the collapse pit is about 1.8m, and its depth is about 2.5m. The karst opening or leaking skylight at the bottom of the reservoir which correspond to the other anomaly areas are relatively smaller on scale. The simple connected experiments of internal and external water and the engineering geological drilling show a good effect of the geophysical survey. This work demonstrates that the self-potential method on the water can be used to investigate of karst geological problems such as the emergency on the water of karst sick reservoirs, and rapid delineation of the range of the shallow karst leakage passages.