The invention discloses an oil well top-blowout prevention device, which comprises a polished rod arranged in an oil well. A shell is arranged on the outer side of the polished rod, a wedging block for fastening the polished rod is mounted at the top end of the shell, a cavity is formed in the shell, a plugging chamber is formed in the position corresponding to the cavity, a ball used for pluggingthe cavity is mounted in the plugging chamber, and a baffle used for preventing the ball body from sliding is mounted in the plugging chamber. The steel ball is blocked through the baffle, during leaking, by pulling out the baffle, the steel ball enters the cavity to block a loop, thus oil gas in the oil well and an oil gas mixture flowing back from a collection and delivery manifold are prevented from blowing out from a wellhead, and resource waste and large-area environmental pollution are avoided.