The slip-zone soil in Shanxi coal-measure soft rock landslide generally contains a large number of clay minerals, which is a typical slip soil with certain expansibility and is different from the conventional slip soil in mechanical properties. In this paper, four typical cases of clayey slip soil in soft rock landslides of Shanxi coal measures in the Yellow River Basin in Shanxi Province are taken as the research object. The research methods include X-ray diffraction test, slow shear test and free expansion rate test. The results show that: (1) The clay minerals of the clayey slip zone soil are mainly composed of montmorillonite, kaolinite, illite and chlorite. The mineral proportions of montmorillonite are 40.95%(DS) , 38.30% (LX) , 34.52% (XN) and 17.65% (ZZ) respectively. (2) The cohesion and internal friction angle of clayey slip soil show two modes with water content (negative index/negative correlation). The content of the montmorillonite that affects the regular pattern is 17.65% ~ 34.52%; (3) The attenuation degree of shear strength of clayey slip soil shows two correlation with the content of montmorillonite minerals(positive correlation/negative). The critical value of the montmorillonite content that affects the degree of attenuation is 34.52%. From the perspective of mineral composition, the attenuation law of shear strength of clayey slip soil is expounded and the unique strength property of this kind of slip soil is revealed.