In order to study the plastic zone and stress distribution of the surrounding rock of circular diversion tunnels, we deduced the analytical solutions of stress field of each zone and the radius of the plastic zone based on elastoplastic mechanics and the unified strength criterion, considering the influence of seepage and strain softening. With the analysis of an actual engineering project, we discussed the influences of the softening of the surrounding rock, seepage and central principal stress coefficient on the tangential stress and plastic zone radius of the tunnel. At the same time, the distribution law of pore water pressure in different directions was analyzed by defining the non-uniform permeability coefficient.The results show that with the change of the non-uniform permeability coefficient, the seepage shows the tendency of anisotropy in different directions, and the closer it gets to the horizontal or vertical direction, the more easily the distribution of pore water pressure is affected by the non-uniform permeability coefficient. The seepage and material softening have different effects on the stress field distribution and plastic zone size of the surrounding rock, and the latter has a more adverse effect on the stability of the surrounding rock. The central principal stress coefficient has a significant influence on the tangential stress and plastic zone of the surrounding rock. Without taking into account the effect of the central principal stress, the calculated results would be relatively small, resulting in the inadequate application of the surrounding rock strength. The conclusion can provide a theoretical reference for the study of the surrounding rock stability during the excavation of tunnels under water-bearing rock masses.