Abstract: Aiming at some restricts and deficiencies of strain-equivalence hypothesis of Lemaitre and through introducing a statistical damage theory,a new statistical damage constitutive model was established based on the curves obtained from tri-axial tests,which could reflect mutual conversion for brittle and ductile properties of rock.On such a basis,through deep discussion of the mechanical mechanism of brittle or ductile properties and their mutual conversion,a new way to determine the conversion condition for brittle-ductile properties was developed by solving differential of pluralistic function.It could simultaneously reflect the combining effects of load condition and mechanical properties of rock itself on brittle or ductile properties and their conversion.The comparison between results of tests and theoretical analysis shows that the method is reasonable and reliable,and it will improve the studies on the constitutive relation of rock.