The invention provides a wellhead blowout preventer. The wellhead blowout preventer is mounted on a blowout prevention box of a polished rod sealer and comprises a body, wherein a polished rod passes through the body; a sealing ball capable of leaning against a polished rod hole in the body is also arranged in the body; a thrusting device leaning against the outer side surface of the polished rod is arranged outside the body; the thrusting device can be used for pushing the sealing ball to lean against the polished rod hole in the body when the polished rod is broken off. The wellhead blowout prevention has the advantages of being capable of blocking the polished rod hole in time when the polished rod in an oil-pumping well is broken off and reducing pollution caused by oil gas which falls on ground.