The deformation and failure mechanism of carbonaceous mudstone interlayered slopes induced by rainfall was revealed. The slope of Lantian Garden in Zhuzhou City was monitored for rainfall, groundwater level, and horizontal displacement of deep soil layers. The groundwater level in different parts of the slope is significantly affected by rainfall, arrange the single hole groundwater level fluctuations in order of magnitude from largest to smallest, namely, landslide trailing edge (fluctuation range of 0.1-1.6 m), slope body (fluctuation range of 0.4-1.1 m), and slope toe (fluctuation range of 0.1-0.6 m). The sliding zone of the slope is located in the interlayer of fully weathered carbonaceous mudstone. Moreover, direct shear tests and numerical analysis of slopes were conducted on carbonaceous mudstones with different water contents. The results showed that as the water content increased, the cohesion and friction angle of carbonaceous mudstones decreased. The increase in moisture content in the interlayer and excavation at the foot of the slope are important reasons for slope deformation and failure. As the moisture content of the interlayer increases, the plastic zone area of the slope gradually increases, and it develops and connects from the foot of the slope to the bottom of the weak interlayer and then to the top of the slope. The maximum horizontal displacement increases with the increase of water content in the interlayer, and the maximum horizontal displacement shifts from the foot of the third-level slope to the bottom of the second-level slope, and then to the weak interlayer. After excavation at the foot of the slope， the changes of plastic zone and maximum horizontal displacement are consistent with those before excavation. However, the increase of plastic zone and maximum horizontal displacement is more significant.