Abstract: Tracer test is an effective and widely used technical method in karst hydrogeology survey. In order to determine the karst aquifer structure, the authors made the tracer test on the main conduit of Hebao underground river in Hezhou of Guangxi, used high-precision online monitoring technology. Fluorescein sodium is used as tracer, and the GGUN-FL30 field fluorescence spectrophotometer is used as online monitoring equipment. The test results show that the underground river between the Entrance (S₁) and the Exit (S₄) is denoted by pipeline flow, and the structure is simple without any other underground water bodies such as a karst lake. The tracer recovery rate is only 63.18 %, indicating that the underground river has other discharge pathways (maybe subsurface flow). Based on Qtracer2 data analysis, the maximum flow velocity of Hebao underground river in dry season is 156.00 m/h and the average flow velocity is 48.09 m/h. Water storage of karst conduit is 26 714 m³. The conduit cross-sectional area is 13.70 m² and the tube diameter is 4.18 m. The conduit system is charactered by dispersion coefficient of 0.505 m²/s, longitudinal dispersion of 37.81 m, friction factor of 0.178, Reynolds number of 48 937, Sherwood number of 1 875.4 and Schmidt number of 1 140. It is shown that karst is well developed and nonuniformity of aquifer is strong in Hebao underground river. So the underground river system is an single conduit system. The groundwater flow is a turbulent flow, which has being bearing pressure in part. The research data and achievements can provide some reference to the numerical model construction of the underground river system.