The invention provides a steam exhausting cylinder of an axially-exhausting steam turbine unit and relates to an exhausting cylinder of a steam turbine unit. The invention aims to solve the problems that a steam exhausting cylinder workshop of a steam turbine unit has high construction cost and the steam exhausting energy is lost when the steam exhausting energy of a radially-exhausting steam turbine unit enters an external condenser from the steam exhausting cylinder. The exhausting cylinder comprises a bearing box assembly and further comprises a water spray mechanism, a smoke discharge mechanism, a shell of the steam exhausting cylinder, an oil path mechanism, a steam-exhausting, flow guide and diffusion mechanism and a steam sealing body of the steam exhausting cylinder, wherein the steam-exhausting, flow guide and diffusion mechanism comprises a second flange, a flow guide cover and two semi-conical diffusion guide flow cylinders; the steam sealing body of the steam exhausting cylinder comprises a steam sealing shell, a high-pressure steam inlet pipe, a micro-negative-pressure steam suction pipe, two first fixing plates and two second fixing plates; and the bearing box assembly comprises a bearing pedestal mounted with a bearing, a bearing box body, a sixth flange and a bearing box protective cover. The steam exhausting cylinder is applied to the steam turbine unit.