Abstract: Formation slip makes casing failure occur frequently after the fracturing of shale reservoirs, which seriously affects construction and production. In order to understand the damage caused by the hydration of shale reservoirs’ structural planes and shear characteristics before and after hydration, shale samples of Longmaxi Formation in southern Sichuan were adopted. Two kinds of regular toothed structural planes of 10° and 40° were prefabricated, and the fault characteristics at different roughness were simulated. Moreover, direct shear tests for hydration were carried out under four normal stresses.The results show that: 1) The structural plane with a high undulating angle of 40° shows plastic deformation characteristics after hydration. The shear displacement–shear stress curve shows the characteristics of stepped rise, and shear stiffness decreases by about 16.3%. However, the structural plane with a low undulating angle of 10° mainly exhibits frictional slip, and the shear stiffness fluctuates slightly. High normal stress (≥ 5 MPa) will aggravate the damage effect of hydration on the structural plane, but the damage promotion effect tends to the upper limit after a long time of hydration (≥ 24 h). 2) At the initial stage of hydration (1–2 h), the shear strength of the structural plane with a high undulating angle of 40° decreases by 13.05% on average, and the overall shear strength decreases by 18.19%; the cohesion and internal friction angle decrease by 16.31% and 16.57%, respectively. In contrast, the shear strength parameters of the structural plane with a low undulating angle of 10° are less affected by hydration, and the fluctuation range is less than 5%. 3) After 360 h of hydration, microporous connectivity and mineral stratification appear on the structural plane with a low undulating angle of 10°, while the structural plane with a high undulating angle of 40° forms large-size tensile fractures, and surface roughness increases. This difference is due to the more concentrated distribution of clay minerals and the more significant hydration softening effect on the structural plane with a high undulating angle, resulting in reduced brittleness and enhanced plasticity. The research results can provide a theoretical basis for shale reservoir fracturing design and casing protection.