The expression of minimum principal stress in triangular mountain under gravity is given by using the analytical solution based on the initial stress in a mountain body. Then, on the basis of the criterion that the internal water pressure should be less than the initial minimum principal stress in a pressure tunnel, the analytical expressions for calculating the minimum slope height, minimum overburden and suitable location of the pressure tunnel are proposed. In general cases, the correspondingly calculated results show that the minimum overburden depends on not only the slope angle but also the slope height. The minimum overburden of the tunnel is independent of the slope height only in the following two cases: one is that both slope angles are? equal, the other is that the two slopes are perpendicular to each other. In the first case, the minimum overburden equals 0.565 6 times of water head of the tunnel, which is consistent with the modified Norway rule.