The slaking behavior is significant effecting the performance of ground fills derived from weathered red mudstone (WRMS). Numerical investigations are used to quantitatively describe the slaking mechanism and slaking modes. A series of petrographic analysis and numerical slaking tests are conducted. The micro-structure of weathered red mudstone (WRMS) is obtained from SEM and XRD analysis, and the slaking process is considered as water induced deteriorative action using the DEM framework. The sedimentary texture and multiple slaking modes of WRMS are considered in this model. The water-rock reaction, porosity distribution, rock resistance and deterioration path are applied in conducted slaking simulation. Typical slaking mode is quantified with slake mode index (Im) including abrasion slaking, breakage slaking and combination slaking. Slaking behavior for 4 types of clay-bearing rocks are analyzed from both macro- and microstate perspectives. Good agreement between experimental and DEM model results are obtained. This study provides insights on mechanical degrading performance of WRMS and similar weak sedimentary soft rocks.