Abstract: In order to study the local infiltration and instability characteristics of the tunnels in Xigeda strata with "Becoming powder because of wind and forming mud owing to water", the indoor soil tests are carried out to analyze the influences of water content on the mechanical properties of the Xigeda strata, and the numerical simulation method is used to explore the deformation characteristics of the surrounding rock of the tunnels in Xigeda strata with change of local water content. The local catastrophe theory is introduced, and the discriminant equation for the water softening stability of the surrounding rock of the tunnels is proposed. The rationality of the results is verified by field monitoring. The results show that the surrounding rock of the Xigeda strata has significant water sensitivity. The mechanical properties of the original rock of the shallow gray shale-sandstone sandstone are most affected by the water content. When the water content exceeds the sensitive limit, the surrounding rock deteriorates rapidly. The mechanical properties show a leaping decline. The increase of the local water content in the circumferential direction of the tunnels in Xigeda strata will lead to the intensification of their convergence deformation. When the water content of the surrounding rock at the vault and the invert arch reaches 30%, the arch settlement and the basal subsidence are obvious. If it is enlarged, the local slaking at the side wall will have a great influence on the horizontal displacement of the surrounding rock, which may cause the surrounding rock to invade the building boundary or even have sudden instability. There are two main modes of longitudinal infiltration of the main inversion: end the cantilever beam modes of end slaking and the simply supported beam mode of the initial branch slaking. In the former mode, at low water content, the displacement of the surrounding rock increases with the proximity of the wetted part. At high water content, the displacement of the surrounding rock increases with the growth rate near the slaking part. In the latter mode, the initial support displacement at the slaking center is most affected by the change of water content, and the vertical displacement of the dome is reduced by about 64.2%. The flat displacement is reduced by about 62.5%, and the vertical displacement of the dome is reduced by about 84.6%. The displacement changes at both sides of the center are different, and the deformation of the surrounding rock near the face is always greater than that in other locations. Based on the water sensitivity of the surrounding rock of the Xigeda strata and the softening plastic characteristics owing to water, the local soft slaking softening mechanical model for the surrounding rock is established, and the derived discriminant equation for stability of the surrounding rock has feasibility and high accuracy in practical engineering application. The research results can provide guidance for the safe construction and stability evaluation of the tunnels in the later Sichuan-Tibet Railway.