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The accidents caused by the instability of bedding rock slope are often encountered in the expressway construction and operation in China. The shear strength parameters of the structural plane play a major role in controlling the stability of the bedding rock slope. In this paper, in-situ direct shear tests were performed on the structural planes containing the thick muddy interlayer, which were widely distributed between the layers of muddy slate in Guizhou Expressway. A numerical model considering the thickness of the muddy interlayer and the shape of the structural plane is established using the UDEC program. Numerical analysis has been performed to reveal the shear mechanical behaviors and the progressive failure mechanism of the muddy structural plane from a mesoscopic perspective. The thickness of the muddy interlayer has a great influence on the shear mechanical behaviors of the structural planes containing the weak layer. With the increase of the thickness of the interlayer, the peak shear strength of the structural plane decreases, and the stress-displacement curve presents a strong nonlinear characteristic. Due to the uneven spatial distribution of the thickness and morphology of the muddy interlayers in actual engineering practices, it should not only adopt the results from in-situ direct shear tests, but also consider the interlayer thickness for the stability analysis of the bedding rock slope. [ABSTRACT FROM AUTHOR]