The invention discloses a wind power generator set and a variable-pitch system thereof. A variable-pitch system of a wind power generator set comprises a magnetic powder clutch, a gear box, a generator, a changeover switch and a variable-pitch motor, wherein an input shaft of the magnetic powder clutch is connected to a rotor unit of the wind power generator set, and an output shaft of the magnetic powder clutch is connected to an input shaft of the gear box; an output shaft of the gear box is connected to a rotor of the generator; and the generator is electrically connected to the variable-pitch motor through the changeover switch. When an external power supply of the wind power generator set fails, the changeover switch is turned on; the input shaft of the magnetic powder clutch is driven by the inertia of blades of the wind power generator set to rotate; the output shaft of the magnetic powder clutch rotates at a predetermined rotating speed; after the rotating speed is increased through the gear box, the generator is driven to generate power; and the variable-pitch motor is pulled by the generator to rotate the blades to a feathering position.