The　cooling　performance　of　the　bypass　cooling　system　in　the　high　pressure　cylinder　of　a　1　000　MW　ultra-supercritical　steam　turbine　was　numerically　investigated　using　conjugation　of　the　flow　calculation　and　heat　transfer　method.　The　temperature　distributions　of　the　inner　case　and　outer　case　of　the　high　pressure　cylinder,　and　the　thrust　equilibrium　piston　were　obtained.　The　comparison　between　the　temperature　distributions　of　the　high　pressure　cylinder　with　and　without　the　bypass　cooling　system　was　performed　under　different　conditions.　The　results　show　that　the　high　pressure　inner　case　and　the　thrust　equilibrium　piston　can　be　effectively　cooled　by　the　bypass　cooling　system.　The　temperature　of　these　solid　domain　decreases　obviously　while　the　outer　case　temperature　increases　slightly.　In　addition,　the　temperature　distribution　of　the　inner　case　is　relatively　uniform　with　the　bypass　cooling　system.