The invention relates to a heat supply steam turbine, in particular to a plain shaft high-back-pressure heat supply steam turbine. The problems that under the heat supply working condition of an existing heat supply unit allowing steam extraction to be adjusted through a butterfly valve, a low-pressure rotor is likely to produce the phenomena of air blasting and heating, and the unit is low in heat efficiency and poor in heat supply steam extraction capacity are solved through the plain shaft high-back-pressure heat supply steam turbine. According to the plain shaft high-back-pressure heat supply steam turbine, a low-pressure rotor is a plain shaft rotor, the upper end of a low-pressure cylinder is provided with two low-pressure steam inlets, and each low-pressure steam inlet is connected with a plug board in a sealed mode; the lower end of a heating steam extraction pipeline communicates with two medium-pressure exhaust ports in the upper portion of a medium-pressure cylinder; the pipe wall of the upper end of the heating steam extraction pipeline is provided with a first steam supply port and a second steam supply port, the first steam supply port is connected with an existing heater for a heating network, and the second steam supply port is connected with one end of a cooling steam pipeline; and the other end of the cooling steam pipeline penetrates through one plug board and communicates with the corresponding low-pressure steam inlet, and a low-temperature pressure reduction device is arranged in the cooling steam pipeline. The plain shaft high-back-pressure heat supply steam turbine is used for heat supply.