The　optimal　position　(lopt)　of　isolating　the　heat　conduction　axially　through　the　oscillatory　tube　wall　is　investigated.　The　maximum　moving　distance　(Lmax)　of　the　contact　surface　inside　the　tube　is　studied　and　a　calculating　formula　of　Lmax　is　derived.　Based　on　the　formula　of　Lmax,　the　influences　of　some　factors　such　as　expansion　ratio,　ratio　of　filling　time　and　discharge　time,　number　of　nozzle,　and　jet　flow　exciting　frequency　on　lopt　are　analyzed.　It　is　shown　that　the　value　of　lopt　predicted　by　the　formula　is　higher　than　that　experiments.　The　formula　of　Lmax　can　only　be　used　to　calculate　the　approximate　value　of　lopt.　The　relation　between　lopt　and　the　axial　temperature　distribution　of　the　tube　wall　is　also　investigated.　Both　the　theoretical　and　experimental　results　show　that　the　position　where　the　axial　temperature　gradient　of　the　tube　wall　reaches　the　maximum　value　is　exactly　the　position　of　lopt.　So　the　value　of　lopt　can　be　determined　simply　by　the　means　of　measuring　the　axial　temperature　distribution　of　the　tube　wall.