The carbonaceous shale is widely distributed in the weak intercalated layer of high mine slopes in southwest China, the stability of which would be affected by the rain infiltration induced delegation of the carbonaceous shale. Carbonaceous shale can be regarded as a typical elastro-plastic material as ductile failure can be observed under complex loading conditions, which is similar with the over-consolidated clays. This paper developed a modified unified hardening model for carbonaceous shale to account for the strain-softening and dilatancy features. Besides, a β-yield function was adopted to better capture the negative dilatancy deformation. Moreover, the proposed model is able to account for the effect of the shape of yield locus on the model predictions, and the model performance can be then improved. Model predictions were compared with the drained triaxial test results of carbonaceous shales, form which a well-agreement was achieved, and consequently the model's performance was verified.