By using the form of new rock micro-unit strength based on Twin Shear unified failure criterion,which satisfies weibull random distribution,a statistical damage constitutive model was developed.The parameters have been gotten by using the method for calculating the extremity of function according to the extremity characteristics of rock deformation curve.Comparison with the result of triaxial compression tests shows that this model can reflect well the relationship between stress and strain of rock.Through the discussion on parameters b、m andF0 in the constitutive damage model,it is found that the change of b has no effect on the pre-peak deformation curve and some effect on the post-peak softening phase;the value of m has some effect on the post-peak brittleness;the value of F0 has effect on the peak stress or rock strength.