Abstract: The present invention provides a controllable compound rotor system for the WECS, with a pilot rotor working under low wind speed conditions and a main rotor working under high speed conditions. By proper switching of the clutch, the WECS can not only achieve a greater starting torque under low wind speed conditions, as well as to capture and utilize the low wind-speed wind energy after being started, but also give full play to the advantage of the main rotor having the excellent wind capturing efficiency under high wind speed and high turning speed conditions. Thus the WECS can capture and utilize wind energy under both low and high wind speed conditions, which greatly expands the speed and zone range wherein the WECS can be applied, facilitating wide application of the WECS.