The invention discloses a locking device for a hatch cover of a manned submersible, and relates to the technical field of machinery. The locking device comprises a rotating shaft arranged on the hatchcover, an outer handwheel, an inner handwheel, a shaft sealing mechanism, a gear transmission locking mechanism, and wedge-shaped strips welded to the inner side of the circumferential wall of a hatch, wherein the gear transmission locking mechanism comprises a ring gear, a pinion, a composite gear and connecting rod systems; the pinion is arranged on the rotating shaft and meshes with the ring gear through the composite gear; and the pinion can drive the ring gear to rotate when rotating with the rotating shaft so as to drive the connecting rod systems to rotate to be in contact with or detached from the wedge-shaped strips and realize the locking and unlocking action. The locking device has a reasonable design of gear transmission and the number of teeth of each gear, so that the locking and unlocking are more labor-saving and the operability is high.