The　coupling　of　the　flow　field　calculation　and　the　conjugate　heat　transfer　method　was　employed　to　investigate　the　cooling　characteristics　of　the　steam　from　the　high　pressure　cylinder　of　a　supercritical　steam　turbine　to　cool　the　solid　parts　of　the　first　stage　in　the　middle　pressure　cylinder　and　the　influence　of　the　cooling　effect　on　aerodynamic　performance　of　the　first　stage.　The　flow　field　and　the　temperature　field　in　the　first　stage　of　the　middle　pressure　cylinder　with　steam　cooling　were　compared　with　those　without　steam　cooling.　The　results　show　that　the　steam　cooling　technology　can　effectively　reduce　the　temperature　of　the　rotor　blade,　blade　root,　rotor　and　wheel　disc　of　the　first　stage.　The　cooling　steam　flowing　through　the　cooling　holes　by　pumping　action　of　the　cooling　holes　has　obvious　cooling　effect　on　the　cooling　holes　and　their　surrounding　area.　The　temperature　of　the　rotor　system　of　the　first　stage　in　the　middle　pressure　cylinder　decreases　by　up　to　80　K.　In　addition,　part　of　the　cooling　steam　enters　the　main　flow　passage　of　the　first　stage　and　mixes　with　the　main　flow,　leading　to　greater　hub　reaction　and　strength　of　the　passage　vortex.