The invention discloses a steam turbine using a novel coal gas supercritical mixed working medium. The steam turbine comprises a high-pressure cylinder, a speed reducer, an intermediate-pressure cylinder, a low-pressure cylinder and a motor. The rotating speed of the high-pressure cylinder is greater than that of the intermediate-pressure cylinder. The high-pressure cylinder is connected with thespeed reducer. The speed reducer is connected with the intermediate-pressure cylinder. The intermediate-pressure cylinder and the low-pressure cylinder are arranged on the same transmission shaft. Theoutput end of the transmission shaft is connected with the motor. The high-pressure cylinder of the steam turbine adopts the high rotating speed design. The problem that a water and carbon dioxide mixed working medium cannot do work in a conventional steam turbine is effectively solved. As the speed reducer is connected with the intermediate-pressure cylinder and the low-pressure cylinder, the number of components needing to be developed is effectively reduced, the research and development time is saved, maximum utilization of energy is achieved, the utilization efficiency of energy is effectively improved, and the steam turbine has important significance on national energy conservation, environmental protection and energy safety.