An air duct includes a top plate, two side plates extending from two lateral sides of the top plate, a baffle plate extending from the top plate and located between the side plates, and a torsion spring connected between the top plate and the baffle plate. The top plate and the side plates cooperatively define an air passage in the air duct. The baffle plate includes a first end pivotally connected with the top plate and an opposite second end. The second end of the baffle plate is rotatable relative to the top plate. The torsion spring includes two arms abutting against the top plate and the baffle plate, respectively. When the baffle plate is rotated relative to the top plate under an external force, the torsion spring resists such force.