Three-dimensional numerical simulation on the double-velocity stage of a steam turbine adopting the design idea of the Curtis stage was carried out.The whole circle flow fields of the two stages were computed and the flow features of the Curtis stage under designed operation were analyzed.Results show that the scattering effect of each nozzle group is not intense after the flow flows out of the Curtis stage nozzle,which only affects one blade passage near the boundary.The main direction of the flow is in the axial direction.When the pressure ratio is relatively larger and the flow velocity becomes supersonic,the circumferencial non-uniform at the outlet of each nozzle group is very serious,the wake aera with low speed and the mainstream area with low pressure appears by turns,so the aerodynamic loads on the rotor blades located at different positions in the nozzle region change greatly.An over-expanded low pressure area occurs in the supersonic nozzle,and the pressure is the lowest in the whole stage,which will strengthen the unsteady effects in the rotor blade passages.