The　prominently　unsteady　effect　of　the　control　stage　in　a　steam　turbine　will　result　in　strong　flow　excitation　force.　On　the　basis　of　the　software　ANSYS　CFX,　a　3-D　unsteady　viscous　model　for　the　control　stage　of　a　steam　turbine　with　a　partial　admission　room　and　a　reinforced　rib　was　constructed　with　the　RNG　k-Ε　turbulence　model,　the　dual　time　step　method,　the　sliding　interface　model　and　the　structural　hexahedron　mesh.　The　simulation　results　show　that　the　time-averaged　unsteady　wheel　efficiency　is　lower　than　steady　wheel　efficiency.　There　are　greater　unsteady　axial　and　circumferential　forces　in　the　inlet　and　outlet　blocked　areas　of　the　rotor　blades.　Spectrum　analysis　indicates　that　the　typical　low　frequency　of　the　exciting　force　is　50　Hz.　The　circumferential　distributions　of　pressure　and　static　entropy　are　nonuniform　and　the　nonuniformity　decreases　gradually　towards　the　downstream　sections.　The　mixing　and　windage　losses　are　larger　in　the　blocked　channel,　leading　to　the　decrease　in　the　blade　work.　In　addition,　the　suction　pressure　of　the　rotor　blade　is　affected　significantly　earlier　than　that　of　pressure　side　when　the　flow　passes　the　blocked　channel.