Abstract：The distribution of effective stress induced by the seepage from pressure tunnel into rocks under a rock slope is estimated by using the image well method proposed by Harr(1962) and the assumptions made by Zienkiewicz(1958) and Fernandez(1994). The rocks are treated as a homogeneous，isotropic，porous，elastic medium. When loaded by its own weight only，the stress distribution in the rocks surrounding pressure tunnel after excavation and water filling is studied. When the pressure tunnel is designed with the Norway rule，the minor principal stress is tensile in the vicinity of tunnel. The tensile area is like a butterfly and it will be extended with the increment of the rock slope angle. The potential hydrosplitting of rocks around pressure tunnel is taken into account and it will appear in the extended orientation of butterfly. If the thickness of tunnel overburden is increased with 1/8 of the minimum one，the tensile area will be decreased effectively. For the pressure tunnel under steep rock slope，the highest slope angle should be chosen very carefully.
