API 16A Annular blowout preventer is manipulated by hydraulic control system. When it is shut off, high pressure oil flows into piston underside closed cavity through the oil port in central shell, to push piston upwards, and the conical surface of piston squeezes rubber core. Due to the restrictions of head cover, rubber core can't go upwards but is pushed towards the center of wellhead until it encloses drilling tool or enclose wellhead totally to shut the well. When well needs to be opened, manipulate the reversing valve of hydraulic control system, so that high pressure oil should flow into the open cavity on top of piston through the oil mouth on shell, to push piston downwards and decompress closed cavity, in which case, the extrusion force on the conical surface of rubber core is removed, and the rubber core is reset gradually with the action of its own elastic force to open wellhead.