Abstract: At present, the theoretical formulas for calculating the external water pressures are mostly for homogeneous surrounding rocks. When there is a strong aquiclude in stratum, the calculated results are larger than the actual pressures, which will lead to the problems of large thickness of lining structures and overestimation of project budget. In order to make the design of lining structures and selection of parameters more reasonable and economical, combined with the deep-buried long diversion tunnel of Dali Section II of Middle Yunnan Water Diversion Project, by using the numerical simulation method and selecting three characteristic points on the outer surface of the linings, the sensitivity analysis of the position height and thickness of the aquiclude in overlying strata and the relative permeability of surrounding rocks are performed, and the variation laws of the external water pressures on linings are investigated. The results show that the external water pressures on the linings decrease with the increase of the height of the aquiclude, but the change is not significant. The thickness of the aquiclude is the key factor affecting the external water pressures on the linings, and the external water pressures on the linings decrease significantly with the increase of the thickness of the aquiclude. The external water pressures on the linings increase with the ratio of the surrounding rocks to the permeability coefficient of the aquiclude. When the relative permeability increases to a certain value, the aquiclude bears most of the head pressures, and the external water pressures on the linings decrease to a certain extent and gradually stabilize. The research results may provide reference for lining design of deep-buried long tunnels with aquicludes in strata.