<p>Assessing the extent of damage on the joint surface is challenging, yet essential for understanding the joint’s shear behavior. We investigate the joint damage evolution and develop a damage evolution equation using the contact change law. The joint samples were examined with direct shear tests at different normal stresses and shear displacements. Experimental results demonstrate that joint roughness is significantly affected by normal stress and shear displacements. Considering these two effects, we obtain the damaged region of the joint surface by transforming the original image of the joint into a binary image under loading conditions. The previously published damage value results and the test results were compiled and analyzed to verify the damage evolution equation. It is shown that the damage evolution equation is applicable to predict the value of the damage to the joint surface with acceptable accuracy. Meanwhile, we establish a shear strength model of joint surfaces through the damage evolution equation. Comparison between the shear strength results obtained from the shear strength model and laboratory experiments demonstrates the validity of the model by comparing the analysis of the error coefficients.</p>