The utility model discloses a polished rod break-off blowout preventer used in producing wells. The blowout preventer comprises a connecting section, a ball sealer, an ejection spring, an accommodating pipe and a plug, wherein, the accommodating pipe is perpendicular to the connecting section and is communicated therewith; the ball sealer and the ejection spring are arranged in the accommodating pipe from the inside out; the plug is in sealed screwed connection with the outer end of the accommodating pipe; and the diameter of the ball sealer is smaller than the internal pipe diameter of the connecting section. When the break-off blowout preventer is in use, the connecting section is connected to the lower part of a packing box, and an oil pumping polished rod moves up and down in the connecting section in a reciprocating manner, so as to pump oil; when the oil pumping polished rod is broken off, the blowout preventer can prevent the generation of blowout; and when the plug, the ejection spring and the ball sealer are dismounted from the blowout preventer, pressure of a pit shaft can be released through the accommodating pipe.