After the steam with a certain pressure and temperature enters the nozzle, the nozzle section shape varies along the steam flow direction, the steam pressure and temperature reduce, the specific volume velocity enlarges and the flow velocity increases, namely the steam expansion in the nozzel is accelerated, the thermal energy is changed into kinetc energy. The steam with higher speed from the nozzle enters the moving blade channels and its direction is changed in the curved moving blade channel. The steam gives impulse force to the moving blade to generate the torque for making the blade rotate to drive the main shaft to rotate and output machanical work, thus the kinetic energy is changed to meachanical energy.