Through the establishment of a nonlinear Nishihara rheological model reflecting the whole process of rock creep, the effects of changes in stress conditions and internal water pressure on the long-term mechanical characteristics of segment lining in a pressure water conveyance tunnel project located in soft rock were studied. The results show that: (1) The radial deformation of segment lining caused by the rheology of surrounding rock increases gradually with the increase of tunnel buried depth. The greater the difference between the initial horizontal and vertical stress values in surrounding rock, the greater the extreme value of radial deformation of segment lining caused by the rheology of surrounding rock. The greater the stress release rate of the surrounding rock, the greater the radial deformation of segment lining caused by the rheology of the surrounding rock. (2) After considering the rheological effect of surrounding rock, the distribution pattern of lining internal force along the ring circumference is basically similar to that of the initial stage, but the values are all changed from the initial stage, among which, the values of axial force are all increased from the initial stage, and after 100 years of project operation, the axial force growth rates of the segment lining for each scheme are between 25.6% and 112.4%, and the average annual growth of the average value of the axial force is between 50 kN/a and 82 kN/a, the extreme value of bending moment is also increased from the initial stage, and its average annual growth is positively correlated with the tunnel buried depth, and the closer the lateral pressure coefficient of surrounding rock is to 1.0, the smaller the average annual increase in bending moment extreme value is. (3) After considering the rheological effect of surrounding rock, the interaction force between segment lining and surrounding rock increases with time, but the extent of increase decreases year by year, and the stress on the lining structure also tends to stabilize gradually. The average annual increase of the interaction force between segment lining and surrounding rock increases with the tunnel buried depth, lateral pressure coefficient, or stress release rate. (4) The differences in the effects of surrounding rock rheology on the long-term stress of segment lining at different internal water pressures are not significant.