Based on the constitutive theory of statistical damage and the assumption that the shear strength of anchor bar’s interface element obeys the Weibull probability distribution, a statistical damage constitutive model considering the residual strength for anchor bar’s interface was established. A small segment of the anchor bar was selected to analyze the force balance, and then, the numerical solution of the shear slip, axial force, side resistance and damage of anchor bar was obtained. Based on an indoor pullout test, the constitutive equation was determined and verified by an engineering example. At the same time, the whole working course for the force transferring of anchor bar was studied as well. Results indicate that with the pulling force increases, the side resistance at the top of anchor bar increases to a peak value and then decays. Finally, it reaches the residual strength remaining unchanged and the shear stress distribution curve is further transferred to the deeper. The essence of the interfacial stress softening is the process of damage’s accumulation and transferring. The parameter m can reflect the softening velocity of shear strength afterpeak value. Thus, decreasing the value of m can reduce the rate of stress softening as well as damage accumulation and transmission, thereby reducing the degradation rate of the overall stiffness of anchor bar and the anchor bar’s displacement in a certain load range.