Abstract: In order to study the shear strength characteristics of the sliding zone soil of the Huixian loess-mudstone landslide under unsaturated conditions, and to reveal the influence of suction, dry density, and vertical pressure, this paper conducts in-depth research based on systematic indoor experiments and theoretical analysis. A four-link unsaturated soil direct shear apparatus is used to conduct consolidated drained shear tests on reshaped loess mudstone samples taken from the sliding zone. The coupling effects of three factors on shear strength are analyzed under 48 different conditions, including 4 suction forces, 3 dry densities, and 4 vertical pressures. The research results show that: (1) Shear strength is significantly positively correlated with the increase of suction and dry density, while cohesion increases nonlinearly with the increase of suction. For every 50 kPa increase in suction, cohesion increases by 20% to 40%, while the internal friction angle does not change significantly with the increase of dry density, with an increase of about 2° to 3°; (2) The shear strength increases linearly with the increase of vertical pressure, and for every 100 kPa increase in vertical pressure, the shear strength increases by 40~65 kPa; (3) Based on experimental data, a modified Mohr-Coulomb strength formula is proposed to quantitatively characterize the synergistic effect of suction, dry density, and vertical pressure on strength. The research results can provide theoretical basis for the mechanical mechanism analysis and stability evaluation of loess mudstone landslides, and provide important parameter support for engineering protection design.