Based on the reflection method, superposition principle and the theory of seepage mechanics, two sim? plified calculation models for external drainage with or without grouting ring are constructed. The calculation formu? las for the inflow of the tunnel and the external drainage tunnel, and the external water pressure expression of the sec? ondary lining of the tunnel are deduced. Analytical degradation verifies the correctness and applicability of the theo? retical model and analytical formula. The parameter sensitivity analysis is carried out according to the deduced for? mula, and finally it is further tested through numerical simulation. The results show that the water inflow of the exter? nal drainage tunnel and the tunnel decreases nonlinearly with the decrease of the permeability coefficient of the grouting circle, and the external water pressure of the secondary lining of the tunnel increases nonlinearly with the de? crease of the permeability coefficient of the grouting circle, but the critical value is less than conventional drainage method; increasing the thickness of the initial support can achieve the purpose of reducing the external water pres? sure of the secondary lining and the amount of water inflow of the tunnel; the reasonable ratio of the permeability coef? ficient for the surrounding rock to the grouting circle, as well as the grouting circle to the initial support, is 15 and 100, respectively, and at the same time, the permeability coefficient of the secondary lining must be strictly con? trolled; the diameter of the drainage tunnel has a significant impact on the inflow of the external drainage tunnel. To protect the ground water resources, the external drainage tunnel diameter is recommended to be 0.2 m.