Abstract: Loess-red bed interface landslides occur frequently in Northwest China, and the critical conditions for their initiation are closely related to the dynamic evolution of the shear strength at the contact interface. To investigate the initiation mechanisms of such landslides, this study takes the slip zone soils of typical loess-red bed interface landslides as research objects. Through ring shear tests and scanning electron microscopy (SEM) analyses conducted on three types of samples (pure loess, pure red bed, and loess-red bed composites) the study systematically evaluates the evolution of shear strength and microscopic structures under varying normal stresses, water contents, and shear rates. The results show that: (1) The peak and residual shear strengths of all three soil types increase markedly with increasing normal stress. The peak shear strength exhibits a unimodal variation with water content and shows a slight increasing trend with increasing shear rate. The loess-red bed composite samples are most sensitive to water content changes, with the most significant strength attenuation. Their brittleness index reaches 15.6%, significantly higher than that of the single-component soils, indicating a stronger tendency toward strain-softening failure. (2) Microscopic observations reveal that increasing water content weakens cementation of clay particles, and the enhanced lubricating of flaky minerals promotes the formation of smooth shear surfaces, leading to a significant reduction in interfacial shear strength. (3) Integrating field investigations with laboratory tests, the initiation mechanism of loess-red bed interface landslides is clarified. The hydraulic weakening effect of the loess-red bed interface is identified as the core mechanism controlling slope instability.