The evolution of sliding zone soil originating from a muddy interlayer in the Three Gorges Reservoir (TGR) in China is accompanied by an increase in clay content, and the subsequent changes in its mechanical properties control the initiation or reactivation of large-scale bedrock landslides. In this paper, using sliding zone soil samples from the Xingfusi landslide in the Wanzhou area, two groups of remolded saturated specimens were prepared with clay contents of 25.0% and 12.5%. Direct shear creep tests and a series of ring shear tests were carried out to investigate the shear behaviors of the specimens with different clay contents. On this basis, the effect of clay content on the evolution of landslides was discussed. The results show that (1) during the long-term creep process before landslide initiation, the shear creep modulus of sliding zone soil in steady creep state significantly decreases with an increase in clay content, which results in the increase in cumulative creep displacement. (2) The increase in clay content in sliding zone soil will lead to a reduction in peak shear strength and residual frictional resistance. In the drained shearing on saturated sliding zone soil with a high clay content, the excess pore-water pressure on the sliding surface cannot dissipate as rapidly due to the small permeability of sliding zone soil with a high clay content, which may favor postfailure movement. (3) With an increase in shear displacement rate, the residual strength first shows a weak “negative shear rate effect” and then shows a significant “positive shear rate effect”. However, with an increase in clay content, the initial shear rate for the residual strength showing a “positive rate effect” will decrease. (4) Sliding zone soil with a high clay content generally does not exhibit strength recovery, while sliding zone soil with a low clay content clearly recovers its peak residual strength due to volume change of the shear zone caused by secondary compression after a long period of reconsolidation. In this case, a greater external load or reduction in effective normal stress needs to be applied to reactivate the landslide.