A　cavity　model　was　built　at　the　outside　of　the　cylindrical　shell　and　contact　pair　was　established　between　the　outer　wall　of　cylindrical　shell　and　the　inner　wall　of　cavity.　The　effect　of　the　contact　and　the　gap　between　the　cylindrical　shell　and　the　cavity　to　the　critical　buckling　pressure　was　studied　by　the　finite　element　analysis　software　ANSYS.　The　result　show　that　the　contact　between　the　cylindrical　shell　and　the　cavity　makes　the　critical　buckling　pressure　of　the　cylindrical　shell　increased　much,　and　the　degree　of　increasing　is　relation　to　the　gap　existed　between　the　cylindrical　shell　and　the　cavity.　The　bigger　the　size　of　the　gap,　the　lower　the　level　of　the　critical　buckling　pressure　increases.