Numerical　investigations　on　the　flow　fields　of　the　internal　bypass　cooling　system　in　the　high　pressure　cylinder　of　a　1000　MW　ultra-supercritical　steam　turbine　were　performed　using　the　flow　calculation　coupled　with　the　conjugated　heat　transfer　model.　The　detailed　flow　pattern,　temperature　distributions,　and　pressure　distributions　in　the　bypass　cooling　system　were　presented.　The　numerical　results　obtained　by　the　flow　and　heat　transfer　method　were　compared　with　those　without　considering　the　heat　transfer　effect　of　the　inner　and　outer　casings.　The　results　show　that　the　steam　flow　field　in　the　bypass　system　is　mainly　influenced　by　the　system　structure　and　the　flow　characteristic　of　the　steam.　However,　the　steam　temperature　field　is　affected　by　not　only　the　steam　flow　characteristic　but　also　the　heat　transfer　effect　from　the　solid　domain.　These　findings　confirm　that　it　is　more　realistic　to　study　the　steam　temperature　field　in　the　bypass　system　with　the　conjugation　of　the　flow　calculation　and　the　heat　transfer　simulation.