In order to study the reinforcing mechanism of carbon nanotubes (CNTs) on the mechanical and thermal stability of nitrile rubber (NBR) O-ring, 1%CNT/NBR and 2%CNT/NBR composite O-ring were prepared by mechanical blending and hot pressing. The mechanical properties of CNT/NBR composite O-ring were tested on the basis of SEM, EDS and FT-IR. The results show that with the increase of CNT content, the elastic modulus and rigidity of CNT/NBR composite O-ring increase. The tensile strength of 1%CNT/NBR O-ring reaches 12.6 MPa due to the formation of C—O bond. The increase of CNT content in NBR matrix can improve the thermal stability of CNT/NBR composite O-ring, and the phase transition temperature of 2%CNT/NBR reaches 297℃. The excellent aging resistance of 1%CNT/NBR composites is attributed to the fact that CNT makes NBR matrix produce CN triple bond functional groups. High-performance CNT/NBR composite O-ring has certain application value in the field of aviation seals.