An anti-backflow valve, including a valve seat and a plug slidingly disposed in the valve seat, is provided. The valve seat has a ring portion protruding inwards at a middle section of a flow passage thereof. The ring portion has a seat portion extended outwards on one side thereof. The plug has an accommodating chamber recessed inwards on one side thereof. The accommodating chamber has an opening radially penetrating through the plug at one end thereof distant from an inlet. The plug has a protruding portion at one end thereof adjacent to the inlet. An outer diameter of the protruding portion is larger than an inner diameter of the ring portion. A spring is provided between the protruding portion and the ring portion, and a portion of the plug penetrating through the ring portion has an O-ring engaged thereon corresponding to the seat portion.