Abstract: Through the hydrology survey of the study area, the groundwater in the tunnel is found to be structural fissure water, the surface water and tunnel water are sampled and analyzed in the wet season and dry season respectively. The hydrochemical characteristics show that the anions and cations in surface water change significantly with the high and low water periods. However, the changes in the content of anions and cations in groundwater are small, and the chemical properties of the water are relatively stable. By comparing and analyzing the water inflow and isotopes of 420 section and reservoir in wet season of 2021, it is speculated that the reservoir water participates in groundwater activities in this section during special hydrological years. However, there are significant differences in the hydrochemical and D, ¹⁸O isotope characteristics between surface water and groundwater in other areas, indicating that the participation of surface water in groundwater activities is not significant. In summary, the groundwater in the study area has a relatively independent aquifer system, mainly supplied by structural fissure water within the rock layers. At ordinary times, bedrock fissure water and brook water supply the reservoir water. When encountering a heavy rainstorm, the reservoir water level will rise above the groundwater level, and then the groundwater will be replenished. It is speculated that when the water level of the reservoir is greater than 535 m, surface water participates in individual groundwater activities in the individual section, but the total amount of water inflow is not large and overall controllable