The utility model discloses a wellhead sealing device for a long-downtime pumping well. The wellhead sealing device is characterized by comprising a hoop head, a tubing pup joint, a tubing coupling and a pipe plug, the tubing pup joint is longitudinally fixed at the upper end of the hoop head, the tubing coupling is fixedly mounted at the top end of the tubing pup joint, the pipe plug is mounted at an opening in the upper end of the tubing coupling, a polished rod sealer is detached from a long-downtime oil wellhead, a pumping rod is placed down, so that a plunger seat is placed at the bottom of a pump cylinder, oil gas does not have leakage points after the device is mounted, the long-downtime oil wellhead is thoroughly sealed, fire accidents such as ignition of an oil well caused by leakage of the oil gas are avoided, and safety of the oil well is ensured.