The study on the division of karst water system is the basis of water balance analysis and water resource evaluation. The Nandong undei^round river is one of the 4 super large underground river systems in southern China. Its hydrogeological conditions are complex and its exploitation potential are also tremendous. Due to the existence of deep grooves in Tertiary in the Mengzi basin of the Nandong underground river system, the groundwater level in this area is higher than that of the north and south sides, and some studies suggest that the water resistance zone is the southern boundary of the Nandong underground river. Through the further collection of hydrogeological and geothermal drilling data inside and outside of the basin, field investigation, cave detection and tracing test, this paper found that the deep grooves do not block deep groundwater, and the southern boundary of Nandong underground river system should be moved to the southern surface watershed area.The groundwater flow in the Mengzi basin and the South should be from south to north as a whole.