In　order　to　study　the　security　problems　of　thermal　shock　caused　by　high　temperature　decomposition　of　ultrahigh　pressure　polyethylene　tubular　reactors,　the　finite　model　of　thick　cylinder　was　established.　The　direct　coupling　method　was　employed,　and　the　temperature,　stress　and　plastic　deformation　of　an　autofrettaged　thick　cylinder　under　fluctuating　internal　pressure　and　thermal　shock　were　analyzed.　The　influences　of　internal　pressure　loading,　thermal　shock　and　internal　pressure　combined　with　thermal　shock　on　residual　stress　of　an　autofrettaged　thick　cylinder　were　discussed.　The　conclusion　suggests　that　the　inner　surface　of　cylinder　was　subjected　to　a　sharp　temperature　gradient　and　large　stress　variation　amplitude.　There　are　small　temperature　and　stress　gradients　in　the　outer　surface　of　cylinder.　The　maximum　equivalent　stress　appears　on　the　inner　wall.　The　residual　stress　is　almost　constant　after　the　pressure　loading　and　changed　a　lot　near　inner　wall　after　thermal　shock.　Thermal　shock　has　a　great　influence　on　the　stress　and　strain　near　the　inner　wall.　The　results　provide　a　certain　basis　for　the　safety　of　the　ultrahigh　pressure　reactor.　©　2016,　Lanzhou　Petroleum　Machinery　Research　Institute.　All　rights　reserved.