Abstract: The Yiluochi karst water system lies in Longyang district, Baoshan City, Yunnan Province. It is on the western margin of the Baoshan basin. The primary aquifer of this system is composed of the sediment of O1y, which is bounded by the clastic rock and the fault. Geological structures with northwest and north-south trends dominate this region. The karst water system has a large karst spring with a flow rate of 440 L/s, as the main source of drinking water in Longyang district of Baoshan City. The proposed Baoshan tunnel of the Darui railway will cross the Yiluochi karst water area. This work has studied the formation of the karst water system and the characteristics of the groundwater flow, in order to provide a scientific basis for the design and construction schemes of the Baoshan tunnel, including the analysis of the influence of the tunnel project on the hydrogeological environment. The applications of chemical major elements and trace elements of the water and isotope analysis, in combination with regional water balance analysis, comprehensively permit to achieve the study objectives planned. The results of water chemistry analysis show the existence of a good correlation between the Yiluochi spring water and the stream water at Shahe river. The water samples of the Chaoyang village at Shahe river and Yiluochi exhibit a significantly higher SO42- content than the other water bodies, which also shows a relationship of the water between Shahe river and Yiluochi. The aluminium (as Al) anomalies of the water samples mostly occur in the water flowing through the Dabaogai syncline core area with the outcrop of O1l stratum, where the Al contents are merely 16.1 ug/L, which is similar to the content of samples from the upstream of the Shahe river. It proves that a little of the Yiluochi water is recharged by that from the syncline O1l stratum.Water balance analysis shows that O1y limestone accepts atmospheric precipitation as a recharge amounts up to 100×104 m3 per year, accounting for a small proportion of total water 1,387×104 m3 discharged via Yiluochi karst springs. It is believed that the Yiluochi springs are fissure water which flows along the limestone karst channels of the limestone interbeds in the Dabaogai syncline. The springs have no hydraulic connection with the upper bedrock fissure water. It is mainly developed in the O1y limestones of the Dabaogai syncline and feeds the Yiluochi springs. The main source of the spring water is the surface water from the upstream of the Shahe river near Chaoyang village, which is located in the northwest of the Dabaogai syncline.