Abstract: To address the issue of a large amount of mudstone powders mixing into the slurry during slurry shield tunnelling in weathered rock layers, this study uses weathered mudstone powders to replace bentonite in different proportions and simulates the process of changes in the properties of slurry. The parameters including slurry viscosity,bleeding rate, Zeta potential, particle gradation, and filtration loss during the filter cake formation process are tested to explore the effect of the mudstone powder mix ratio on slurry and filter cake properties, and the mechanisms of the changes in properties are analyzed. The results show that the addition of mudstone powders significantly reduces slurry viscosity, increases the bleeding rate, causes particle agglomeration in the slurry, and lowers the Zeta potential.When the mix proportion of mudstone powders is 30%, the filter cake becomes denser, and compared with pure bentonite slurry, the permeability coefficient decreases by 70.30%, and the porosity decreases by 43.94%. Under a pressure of 0.45 MPa, the air-tight time exceeds 320 minutes. However, when the mudstone powder mix proportion exceeds 30%, the properties of the filter cake decline significantly. The reduction of Zeta potential in slurry when mud? stone powder is added is a key factor in the increase in particle size, the increase in bleeding rate, and the decrease in air-tight time and air-tight value in the filter cake.