Abstract: The influence factors of external water pressure of lining and water inflow are important issues in the design of diversion tunnel in complex karst region, which are of great significance to tunnel drainage and environmental protection. In view of the Dali section Ⅱ diversion tunnel of the water diversion project for the central area of Yunnan Province, the formulas of external water pressure reduction coefficient and tunnel water inflow were derived from theory of circular tunnel seepage flow. The influence of lining and grouting ring parameters on external pressure and water inflow of tunnel lining was studied respectively. The results show that the increasing thickness and the decreasing permeability coefficient of lining both can lead to the decrease of external water pressure, but the tunnel water inflow will increase significantly, which is unfavorable for environmental protection around the tunnel. However, increasing the thickness of the grouting circle or reducing its permeability coefficient can effectively control the water inflow of tunnel while reducing the external water pressure of the lining. The research results can provide references for the design of waterproof and drainage system of tunnel engineering in complex karst area of Southwest China.