Statistical damage mechanics is an effective method by inversing constitutive relation based on the data of triaxial experiment in rock failure. Taking the Hoek-Brown damage criterion as the distribution variab and taking advantage of the strain-equivalence hypothesis, a rock damage constitutive model is established under triaxial stress state based on the strength characteristics of rock micro-unit with power function distribution. Then the constitutive model is verified by test data of triaxial experiment and the result indicates that the proposed constitutive model can reflect the relationship between stress and strain of rock and the process of rock failure accurately. So, the proposed constitutive model is rational and feasible.