Aimed at the failure of the O-rings caused by thermal expansion and stress relaxation under the high temperature working condition,both the stress and its time varying relaxation of O-rings were studied by taking and not taking the temperature into consideration.The reliability degree of shear failure,sealing failure and multiple failure was discussed by using the response surface method.The results indicate that high temperature has a great influence on the stress distribution and relaxation speed of O-rings.The reliability degree of shear failure and sealing failure is in a downward trend with time,which is decreased rapidly at the initial stage and then decreased slowly to be stable.The reliability degree of multiple failure is decreased sharply at the beginning and then becomes steady,indicating that the O-rings will have a low probability of failure if they are judged not fail in the early stage.