Abstract: It’s not avoidable of the concrete material to produce micro-fractures and micro-hole at the shaping process. In the light of this feature, the author defines a negative damage variable based on the concrete elastic modulus, and introduces the negative damage variable to the damage constitutive equation. The relation between the elastic modulus of the prestressed concrete beam with various prestressing force is obtained by analyzing the dynamic behaviors of the externally prestressed concrete structures using the damage mechanics and load finite-element method. The results indicate: the damage theory with the negative damage variation based on the concrete elastic modulus can predict well the variation of the concrete elastic modulus, especially at the stage in application of pre-stress; the natural frequency determined by the elastic modulus in the corresponding state is consistent with the test results in trend, and the error is acceptable.