Abstract: A hydropower station in southwest China is located in the middle part of the fold belt between the eastern Yunnan platform and the western Guizhou platform on the Yangzi platform. The largest fault in the near field area is F38, which is 61 km long and generally protrudes to the northwest in an arc shape. The dam site is located in the east-west structural belt on the south side of F38 fault, which is the core area of the northeast end of Dashuitang oblique anticline with an axial direction of NNE. At the dam site, the river flow direction is 25°-30°, and the valley is basically in symmetrical V shape. The dam area is dominated by carbonate rocks, and faults and karst are relatively developed. There are karst funnels, depressions, sinkholes and other surface karst forms in the upper level of planation surface on both sides of the dam site. Early investigation results of adits, drilling, foundation pit excavation and geophysical prospecting show that there are some karst caves, trough-like caves and interlayer corrosion in the riverbed and both sides, which are interconnected according to their extension trend. Since the reservoir starts to store water, continuous leakage has occurred in the 1,315 m drainage tunnel of the resistance body on the right bank of the dam site. With a rapid rise of the reservoir water level, a large number of muddy fillings gush out from each water inflow point. Then the water gradually becomes clear, the leakage gradually increases to about 1.9 m³·s⁻¹, and the reservoir cannot store water normally. In order to find out the leakage path of reservoir water and take measures to reduce leakage, karst leakage research is carried out. 15 boreholes are arranged on the right bank of the dam site, with the borehole bottom elevation from 1,133.3 m to 1,249.9 m, and the borehole water pressure tests of 121 section are carried out below the bottom elevation of the dam foundation curtain. Results show that the permeability area of Lugeon value q≥3 Lu below the curtain bottom elevation is distributed in a band, mainly between 1,200 m and 1,260 m, where Lugeon value gradually decreases from q=3-75 Lu to 2-8.82 Lu toward the right bank. Inspection holes are arranged along the grouting curtain line of the grouting drainage tunnel at the elevations of 1,293 m and 1,340 m on the right bank, and the bottom elevation of the holes is about 1,200 m. The water pressure test, borehole television, the data of simple water pressure test and abnormal records during the grouting process are analyzed comprehensively. 8 inspection holes of the 1,293 m grouted drainage tunnel can be grouted normally, and no hydraulic connection with the water gushing channel of the 1,315 m downstream drainage tunnel is found. 6 inspection holes are arranged in the 1,340 m grouting drainage tunnel, and hundreds of grouting holes are arranged at intervals of 10 m to the left and 2 m to the right of borehole inspection-4. There are 2-3 rows of grouting holes in local sections, and the abnormal tunnel sections are densely grouted. The exploration results of 1,340 m grouting drainage tunnel show, (1) There is no large structural fracture zone developed in the left of inspection-4. Grouting is successfully completed in each hole, and no channel connected with water inrush at 1,315 m is found. (2) For the multiple boreholes between inspection-4 and inspection-3, porous water loss occurs when the drilling is at the elevation of 1,245-1,270 m, and the water pressure cannot be added to the predetermined pressure in the test. After the valves are closed at each water outlet of 1,315 m drainage tunnel, the water gushing phenomenon appears in many holes, especially holes near the right side of the inspection-4, which indicates that there is a leakage channel for reservoir water to bypass the curtain line at the inspection-4 position, and there is a strong hydraulic connection with the downstream 1,315 m drainage tunnel. (3) Most boreholes between inspection-3 and inspection-1 are drilled after the valves of water inlets of the 1,315 m drainage tunnel are closed. In the tunnel section of about 20 m from the left of fault F₁₂ to inspection-2, the probability of borehole water inflow is relatively high when the drilling reaches 1,220-1,260 m, where there is a corrosion zone. Especially in the area 8 m from the left of fault F₁₂, the water inflow phenomenon occurs in almost every borehole between the elevation about 1,240-1,260 m, which shows that the section from the left of fault F₁₂ to inspection-2 is the main channel for the reservoir water to leak around the curtain to the 1,315 m drainage tunnel downstream. According to the water inflow conditions of each inspection hole and grouting hole of the 1,340 m grouting drainage tunnel, combined with the geological characteristics of the right bank resistance body, exploration results and rock permeability test results, the karst leakage path of 1,315 m water inflow is judged as follows: reservoir water infiltrates through the fault fracture zone F₁₂, reaches the dissolution zone of the deep rock bedding plane, and then gradually lifts along the dissolution zone at a dip angle of 30° downstream, and bypasses the curtain line at the elevation of 1,220-1,260 m near inspection-4 and from the fault F₁₂ to inspection-2. After encountering the geological weak zones such as vertical solution grooves, solution ditches and solution caves, reservoir water of the local area seeps upward and finally gushes out from the geological weak point of 1,315 m drainage tunnel. The treatment of karst leakage in this project is a dynamic interactive process of investigation and grouting. With the measures of grouting curtain reinforcement, the main leakage channel is blocked, and the reservoir water leakage is effectively controlled. The leakage of the right bank 1,315 m corridor gradually reduces to about 38 L·s⁻¹. There is basically no water dripping on the tunnel wall, and the treatment meets the design requirements.the treatment meets the design requirements.e design requirements.