Abstract: Using HTHP dynamic autoclave and self-designed O-ring compression simulator, the effects of compression state and medium phase state on the mechanical properties of terafluoethylene-propylene rubber(FEPM) O-ring were investigated in the condition of high temperature(140 ℃), high pressure(30 MPa) and high content of CO₂(0.5 MPa). Then, the tensile, hardness and compression permanent deformation of the O-ring sample were taken as the investigation index, and the damage regular of the mechanical properties before and after the corrosion were analyzed. The results show that the mechanical properties of FEPM O-rings decreased in simulated wellbore condition. The corrosion of FEPM under the free state was more serious than that under the compression state and the corrosion was more serious in liquid phase than in gas phase, the experimental results show a preferable regularity. The mechanical properties reduction degree of the rubber material in compressive stress state could be taken as the main reference for the reliability of rubber seal and the applicability of the rubber material.