In　this　study,　an　experimental　investigation　was　carried　out　to　explore　the　heat　transfer　characteristics　of　the　smooth　water　wall　tube　of　an　ultra-supercritical　circulating　fluidized　bed　(CFB)　boiler.　The　ranges　of　the　test　pressure,　mass　flux,　and　heat　flux　were　23–32　MPa,　550–1200　kgm−2s−1,　and　200–560　kWm−2,　respectively.　The　material　of　the　tube　used　in　the　test　was　12　Gr1MoVg.　The　diameter　and　wall　thickness　were　30　and　5.5　mm,　respectively.　The　length　of　the　test　section　was　2　m.　The　effects　of　the　pressure,　mass　flux,　heat　flux,　buoyancy,　and　flow　acceleration　on　the　heat　transfer　characteristics　were　analyzed.　The　formulas　of　the　heat　transfer　coefficient　were　fitted,　and　the　existing　classical　formula　was　used　to　evaluate　the　experimental　data.　The　mechanism　of　heat　transfer　enhancement　and　deterioration　of　the　tube　were　also　investigated.　Results　showed　that　at　the　area　of　supercritical　pressure,　the　wall　temperature　gradually　increased　with　the　increase　of　enthalpy　in　the　pseudo-enthalpy　region　and　sharply　increased　with　the　increase　of　enthalpy　in　the　low-enthalpy　region　(enthalpy−1)　and　high-enthalpy　region　(enthalpy＞2400　kJ　kg−1).　This　phenomenon　indicated　that　heat　transfer　enhancement　occurs　near　a　pseudo-critical　point.　The　increase　of　heat　flux　resulted　in　rapid　heat　transfer　deterioration.　Thereafter,　the　wall　temperature　rose　immediately.　The　deterioration　was　delayed　with　the　increase　of　mass　flux　and　pressure.　The　effect　of　buoyancy　and　flow　acceleration　on　the　heat　transfer　concentrated　on　the　pseudo-critical　temperature　of　the　fluid.　Among　the　five　selected　heat　transfer　correlations,　the　Jackson　and　Bishop　correlations　agreed　well　with　the　experimental　data.　Copyright　©　2018　ASME.