Abstract: The weak interlayer is a delicate geological formation that significantly influences the stability of water and hydropower engineering slopes. This study aims to investigate the shear mechanical characteristics and failure mechanism of the weak interlayer under dry-wet cycle conditions in the reservoir bank slope. The research focuses on the left bank shoulder slope of the Caizi Dam pumped storage power station lower reservoir dam site. Real rock morphology characteristics are replicated using 3D printing technology to create structural surfaces, and the roughness coefficient JRC of different structural surfaces is calculated. Subsequently, shear tests are conducted on weak interlayers containing artificial joint surfaces subjected to dry-wet cycles. The research findings indicate that, after multiple dry and wet cycles, the shear strength decreases exponentially as the number of cycles increases, and eventually approaches a very low stable value, which is merely 5% of the initial strength. Moreover, the shear strength is positively correlated with the roughness of the structural plane. Shear stress-strain curve of the weak interlayer exhibits a step-like decrease during the failure stage. The magnitude of this decrease in the residual curve is directly proportional to the roughness of the structural surface. Additionally, samples with higher roughness on the structural surface will reach the residual strength stage at a shorter shear displacement. Furthermore, the residual strength will decrease as the number of dry-wet cycles increases. These research findings hold significant value for the stability evaluation of slopes in energy dissipation and water storage power stations.