Cement sheath integrity supports the long-term safety production of oil and gas well. In order to research failure mechanism of cement sheath in HPHT conditions, uniaxial and triaxial compressive tests and cyclic loading test of cement are conducted, and mechanical property and failure characteristics of cement are analyzed by using statistics damage mechanics. A simple formation model and an interbedded one are built in FLAC 3D Software to analyze the damage characteristic and the influence factors of cement, and numerical results show that: with the increase of wellbore pressure, elasticity modulus of formation and cement, and non-uniformity coefficient of ground stress, the damage of cement will increase; During cyclic loading and unloading, damage of cement will increase with cycle time, but most of the damage will occur in the first cycle; The reduce of pore pressure will aggravate the damage, especially in the border between different formations. Damage mechanics can give a better way to evaluate cement sheath integrity in the life of a well.

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