Reservoir leakage can cause a waste of precious water resources and even severe environmental consequences. In this study, we use continuous resistivity profiling to evaluate the leakage problem of the Shuangwangcheng reservoir along the east route of the South-to-North Water Diversion Project. A numerical example was first built to validate the method's effectiveness in a saline aquifer environment. Thirty-five waterborne survey lines were then conducted with a total length of 74 km, and two ground survey lines had a length of 1.27 km each. We evaluated the quality of the overall data with the apparent resistivity of intersection points. Based on ground survey results, the resistivity value larger than 2 Ω.m at the bottom of the reservoir is regarded as leakage areas. Therefore, we divide resistivity survey results into three zones: freshwater reservoir, leakage zone, and saline aquifer. The distribution of freshwater intrusion is evaluated by fence diagram and interpolated horizontal resistivity contour maps. The delineated leakage zone is consistent with the lack of a low permeable loam layer on the north and east parts of the reservoir. The results prove that the waterborne resistivity survey method can efficiently and effectively assess leakage distribution inside a reservoir.