Abstract: The planned Ronglai reservoir is located on a karst platform between the Beipanjiang and Nanpanjiang water systems in the Pearl River basin, southwestern China. There are low-lying troughs on the left and right banks of the reservoir and large karst lakes with a number of sinkholes develop at the downstream of the chosen dam site. To further examine geological conditions of this reservoir, this work conducted the studies in terms of hydrogeological surveys, drilling, geophyical exploration and connectivity tests, involving the analyses of terrain, strata, lithology, geological structure, hydrogeological settinggy, and karst characteristics. Analytical result suggests that there will be no water leakage to occur at the bottom of the reservoir basin because of the existing impermeable rock mass. But as the two banks of the reservoir area are affected by the low groundwater level of the watershed, fault fracture zones and karst development, potential leakage remains a problem. Meanwhile, leakage can also take place along gentle-dipping rock surfaces and erosion cracks on the banks of the reservoir head. It is recommended to adopt the vertical anti-seepage method, in which the impervious standard (water permeability) is less than 3 Lu or the lower limit of the anti-seepage curtain is 10 m below the stable groundwater level. At the same time, when encountering geological defects such as caves, the threshold of this measure should be lowered to a appropriate extent.