The invention discloses a last-stage humidity detection device of a steam turbine and the steam turbine. The last-stage humidity detection device of the steam turbine comprises a probe, a sleeve, a louver, a communicating pipe and a control device, wherein one end of the communicating pipe is communicated with the sleeve, and a cover plate is arranged at the other end and exposed from a cylinder.When the device is in use, the frequency value at the time is measured firstly in the closed state of the louver; then, the louver is opened by the control device, so that steam enters from the frontend of the sleeve and passes through the probe, and the frequency value in the state is measured; finally, the real-time final-stage humidity of the steam turbine is calculated based on the two measured values. Movement of the probe is not needed in the process, that is, a guide rail is not involved, jamming of the rail is avoided, and loosening of the probe is prevented. Therefore, the last-stagehumidity detection device of the steam turbine can prevent the probe from loosening while avoiding rail jamming, and problems in the prior art are solved.