Considering the change of cement sheath elastic modulus on casing strength, whether is helpful or not, by the elastic mechanics mathematical method, the effect of cement sheath elastic modulus on casing external collapse load was researched on the conditions of uniform in-situ stress on different properties in formation. The study shows that in a formation with soft lithology, the cement sheath shall have higher stiffness to help casing withstand a part of load; while in a formation with hard lithology, the stiffness of the cement sheath shall be reduced in order to protect casing. The ideal property of concrete sheath should be of high strength and low stiffness. The high strength of concrete sheath could resist higher loads of formation stress. At the same time, the low stiffness of that could reduce the transfer coefficient of stress greatly. Increasing the thickness of the cement sheath is not always reducing the stress of the casing. When the elastic modulus of cement sheath achieves some value, increasing the thickness of cement sheath could decrease the casing stress. From this point, the property of cement slurry shall be adjusted according to formation conditions in the process of cementing to reduce casing load and achieve the best cementation.