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In water-rich karst area, the lining structure of diversion tunnels is prone to cracking and gushing water problems under high water pressure, so water drainage design is often adopted for these diversion tunnels, that is, to drill holes on the lining structure and install drainage equipment to lower the external water pressure of the lining, therefore, accurately calculating the external water pressure of the lining is a very important work for the safety of the lining structure of tunnels. Based on the method of equivalent permeability coefficient, this paper regarded the drainage capacity of drainage equipment installed on the lining and the permeability of lining itself as a whole, and used the virtual equivalent permeability coefficient of the lining to describe it; then, this paper proposed a method for determining the equivalent permeability coefficient of lining and used it to analyze the drainage effect. Main conclusions of this paper are: 1) By designing tests for the drainage model of the tunnel drainage equipment and theoretically analyzing the seepage field of hydraulic tunnel, the equivalent permeability coefficient of the lining was determined and its accuracy was verified via numerical calculation; 2) Analytical calculation results and numerical simulation results of the tunnel were analyzed, and their conclusions about the effect of drainage capacity on the external water pressure of the lining were consistent; 3) The tunnel drainage design is an effective measure to reduce the external water pressure of the lining, the pressure reduction effect is obvious when k<1×10-6, and the groundwater level around the tunnel could be reduced at the same time. This paper provides a useful evidence for the design of high-pressure water diversion tunnels. [ABSTRACT FROM AUTHOR]