In　this　paper,　numerical　investigations　on　the　aerodynamic　characteristics　of　the　last　three　stages　and　the　exhaust　hood　for　a　large　power　steam　turbine　were　conducted　under　a　series　of　mass　flow　conditions　(100%　similar　to　10%　of　the　design　condition)　using　the　commercial　CFD　software　ANSYS-CFX.　The　single　passages　of　the　last　three　stages　and　the　whole　exhaust　hood　are　combined　together　as　the　computational　domain.　The　main　objective　of　this　present　work　is　to　analyze　the　aerodynamic　performance　and　the　flow　behavior　of　the　exhaust　hood.　The　variations　of　the　static　pressure　recovery　coefficient　and　the　total　pressure　loss　coefficient　while　the　mass　flow　rate　decreasing　were　analyzed.　The　static　pressure　distributions　along　the　diffuser　surface　under　different　flow　conditions　were　illustrated.　The　development　of　the　vortex　near　the　outlet　of　the　diffuser　was　demonstrated　through　the　velocity　vector　distribution　at　the　meridional　plane　of　the　exhaust　hood.　The　windage　conditions　were　analyzed　under　20%　and　10%　mass　flow　rate　of　the　design　condition.　In　addition,　the　back　flow　phenomenon　was　observed　when　the　mass　flow　rate　was　below　50%　of　the　design　condition,　and　it　starts　from　the　hub　region　of　the　last　stage　rotor　and　grows　up　along　the　radial　direction.　The　back　flow　also　induces　a　sharp　turning　on　the　span-wise　distribution　of　the　angle　theta　(defined　in　Fig.　9)　at　the　outlet　of　the　last　stage　rotor.　The　three-dimensional　streamlines　inside　the　exhaust　hood　under　different　mass　flow　conditions　were　also　compared.