Abstract: The reasonable calculation of the external water pressure on the tunnel lining is the focus of the design of the diversion tunnel in the karst area, which has a great influence on the selection of structural design parameters and the operation safety of the hydropower station. In view of the Dali section Ⅱ diversion tunnel of the water diversion project for the central area of Yunnan Province, the ABAQUS numerical simulation method is applied to studying the variation law of the external water pressure and seepage field of the tunnel lining under the conditions of different lining parameters and grouting circle parameters so as to determine the appropriate design parameters. The analysis results indicate that the external water pressure is basically considered to be equivalent to the hydrostatic pressure at this point when the tunnel lining is fully blocked, and the water pressure reduction cannot be considered in this study. The external water pressure on the tunnel lining can be extremely reduced by increasing the lining permeability coefficient, but it has a great influence on the seepage field. Reducing the permeability coefficient or increasing the thickness of the grouting circle can not only reduce the external water pressure on the lining but also weaken the disturbance to the seepage field, which can greatly alleviate the contradiction between drainage decompression and environmental protection. The research results can provide some references for the design of the waterproof and drainage system in the tunnel engineering in the complex karst area in the southwest China.