<p>The mechanical behaviors of rock joints plays a vital role in controlling the stability of layered rock slopes. Most existing constitutive models for jointed rock masses assume a constant initial roughness, ignoring its evolution during shearing and leading to inaccurate descriptions of shear deformation. To address this limitation, a new constitutive model for jointed rock masses is proposed, which explicitly considers the dynamic evolution of joint surface roughness based on the stress–displacement characteristics obtained from direct shear tests. The proposed model quantitatively links shear displacement with joint roughness degradation, providing a more accurate representation of shear stiffness evolution under loading. The proposed joint model was implemented into the numerical software and validated using laboratory shear test data under different normal stresses, showing close agreement in both peak and residual strength. Furthermore, slope stability simulations demonstrated that this model can realistically reproduce the progressive failure process and yield higher accuracy in predicting safety factors than the traditional Mohr–Coulomb joint model. The results confirm that incorporating roughness evolution enhances the reliability of numerical analysis for the deformation and failure of layered rock slopes.</p>