Abstract: Significant changes of the wellbore pressure and temperature in HPHT wells may bring great challenges to the mechanical integrity of the cement sheath.On the basis of the multi-layer thick wall cylinder theory of elastic mechanics,this paper proposed a mechanical theoretical model of cement sheath under both pressure and temperature,and investigated the distribution of radial and tangential stresses within cement sheath.Results of studies demonstrate that when the sustained casing pressure(SCP)is 10-50MPa and wellbore temperature change(WTC)is -50-50℃,the cement sheath is generally radially compressed and tangentially tensioned,and the maximum compressed and tangential stresses are both at the inner wall of the cement sheath.When the WTC keeps constant,both stresses increase with the increase of the pressure inside the casing.When the SCP is constant,both stresses increase with the increase of the WTC and decrease with the decrease of the WTC.A wellbore temperature increase will aggravate the possibility of tangential tensile failure in cement sheath,and a wellbore temperature decrease will lead to its failure due to radial tension.Consequently,wellbore temperature changes should be considered for their effect on the stress of the cement sheath,and the cement with lower elastic modulus and higher bond stength with casing and formation should be used to enhance the comprehensive mechanical performance of cement sheath in HPHT wells.