When wind speed is less than or equal to rated value, its blades rotating around axle may automatically turn to be parallel with wind direction in contrary wind area or to be perpendicular to wind direction in the area where it is against the wind. When wind speed is greater than rated value, the blades may be automatically changed in its inclination to realize stepless decreasement of area against the wind. Thus, its output power and rotational speed are kept constant. Its advantages include low output fluctuaction, high wind speed to be resisted, low starting wind speed, and automatic or manual stoppage during heavy wind.