Abstract: Aiming at the failure in the seal integrity of cement sheath under the variable internal casing pressure, based on the theory of continuous damage mechanics, this paper describes the complete stress-strain mechanical properties of actual set cement using the constitutive model of plastic damage that considers both set cement damage and the post-yield stress-strain relationship, and studies the effect of the mechanical properties of set cement on seal integrity. The results show that adding elastic-ductile materials into conventional cement slurry to form elastic-ductile cement will reduce the elastic modulus of cement sheath, which can prevent the formation of micro gap and reduce the size of micro gap, as well as prevent and reduce the tensile damage of cement sheath. However, it does not mean that the lower the elastic modulus, the better the seal integrity of elastic-ductile cement. The seal integrity is also related to the tensile and compressive elastic deformation performance of cement sheath. Under the same circumstances, the greater the maximum tensile and compressive elastic strain of cement sheath, the more impossible the failure in seal integrity. It is recommended to evaluate the seal integrity of cement sheath comprehensively based on its elastic modulus and the complete stress-strain relationship under tension and compression. The research results provide a basis for evaluating the seal integrity of cement sheath and optimizing the mechanical properties of set cement.