Abstract: A karst hyporheic zone is an important channel connecting a karst aquifer and a river, which plays an important role in protecting the ecology and water quality in both of river water and groundwater. In order to study the geochemical characteristics of the karst hyporheic zone in the middle reach of the Zuo River, water temperature, dissolved oxygen, pH and electrical conductivity of 21 water samples from the Zuo River (5 samples), ground water (3 samples) and motor-pumped well (13 samples) were measured. The spatial distribution characteristics and control factors of the karst hyporheic zone on both sides of the Zuo River are discussed by applying the flow system theory of karst groundwater. The results show that the the electrical conductivity and concentrations of Ca²⁺ and HCO₃^- of the Zuo River are lower than those observed in the motor-pumped wells in the karst hyporheic zone. On the contrary, the pH values, water temperature and dissolved oxygen in the Zuo River are higher than those observed in the motor-pumped wells, which shows a hydrogeochemical gradient, including a dissolved oxygen gradient, a pH gradient, a temperature gradient, and Ca²⁺ and HCO₃^- gradients in the Hyporheic zone. The recharge-runoff area in the left bank of the Zuo River is large. Due to the larger karst water flow system, the karst is very developed, and the karst pipeline system is formed. Within the right bank, the interstream block is surrounded by the Zuo River. The basin area is about 6 km², due to the smaller karst water supply area, the karst development depth and intensity is less than that of the left bank. The boundary of the hyporheic zone is more than 1 000 m far away from the river bank in the left bank, whereas the boundary of the hyporheic zone is less than 200 m away from the river bank in the right bank, and the former is five times more than the latter. This study is helpful in the utilization of groundwater and the protection of water quality along the Zuo River