This　paper　researches　the　steam　cooling　performance　applied　in　ultra-supercritical　steam　turbines.　The　inlet　main　steam　temperature　of　the　middle　pressure　cylinder　is　the　same　or　higher　than　that　of　the　inlet-main　steam　of　the　high　pressure　cylinder　due　to　the　reheated　technology　used　in　the　ultra-supercritical　steam　turbines.　The　steam　cooling　design　is　used　to　decrease　the　thermal　fatigue　and　high　temperature　creep　of　the　rotor　in　high　temperature　operation　environments　for　the　ultra-supercritical　steam　turbines.　The　conjugation　flow　and　heat　transfer　method　was　applied　to　investigate　the　cooling　characteristics　of　the　first　two　stages　solid　parts　including　the　rotor　blade,　wheel　disc　and　rotor　blade　root　of　the　middle　pressure　cylinder.　In　addition,　the　influences　of　the　cooling　steam　on　aerodynamic　performance　parameters　distribution　of　the　first　two　stages　were　also　analyzed　by　comparison　of　the　through　flow　calculations　without　steam　cooling.　The　results　show　that　the　steam　cooling　design　can　effectively　cool　the　rotor　system　of　the　first　stage,　and　the　temperature　of　the　solid　part　near　the　cooling　hole　decreases　by　up　to　30K.　The　temperature　of　the　steam　flowing　through　the　diaphragm　seal　of　the　second　stage　increases　and　is　higher　than　that　of　the　local　main　flow.　Therefore,　the　temperature　of　the　rotor　system　of　the　second　stage　is　relative　high.　Furthermore,　the　cooling　steam　enters　into　the　through　flow　passage　and　mixes　with　the　main　flow.　This　flow　behavior　leads　to　changes　of　the　flow　pattern　and　aerodynamic　parameters　distribution　near　the　blade　hub　regions.