The　aerodynamic　performance　of　a　nozzle　controlling　stage　contributes　significantly　to　overall　efficiency　of　a　large　power　steam　turbine.　The　partial　arc　admission　flow　condition　results　in　the　non-axisymmetric　inlet　flow　condition　for　the　nozzle　controlling　stage.　The　commercial　CFD　software　NUMECA　is　utilized　to　solve　the　RANS　for　investigating　the　flow　fields　of　the　nozzle　controlling　stage.　The　multi-block　structural　grid　is　used　to　generate　the　computational　grid.　The　finite　volume　method　is　applied　to　discretize　the　RANS　equations.　The　Baldwin-Lomax　turbulence　model　is　used.　Three　flow　conditions　of　the　four　valves　opening,　three　valves　opening　and　two　valves　opening　of　the　nozzle　controlling　stage　were　conducted　with　thinking　of　the　non-axisymmetric　inlet　boundary　condition　according　to　the　partial　arc　admission.　The　numerical　results　show　that　the　efficiency　decreases　according　to　the　part　arc　admission　for　four　valves　opening　condition.　As　to　the　three　valves　opening　and　two　valves　opening　cases,　the　efficiency　decreases　in　further　according　to　the　larger　part　admission.　In　addition,　the　flow　rate　of　the　nozzle　controlling　stage　increase　with　the　outlet　pressure　decreases　and　so　does　output　power.　The　present　work　provides　the　theoretical　basis　and　technical　support　to　improved　design　governing　stage　for　steam　turbines.