The invention provides a flat nailing stapler, and belongs to the technical field of machines. The flat nailing stapler solves the problem that an existing flat nailing stapler is poor in nailing stability. The flat nailing stapler comprises a base, a gland and a nail box located between the base and the gland; the back end of the nail box is hinged to the back end of the base, the back end of the gland is hinged to the back end of the base, a nail plate capable of swinging vertically relative to the base is arranged between the base and the nail box, and a flat nailing opening is formed in the front end of the nail plate; a sliding plate is arranged on the base, the back end of the sliding plate is provided with an upwards-protruding push part, and a push rod is arranged on the inner side of the gland; the lower end of the push rod extends out of the gland and forms front-and-back abutting with the push part of the sliding plate, and the upper end of the push rod is hinged to the inner side wall of the gland; and a swing rod is arranged between the side portion of the front end of the sliding plate and the side portion of the front end of the nail plate, the lower end of the swing rod is hinged to the front end of the sliding plate, the upper end of the swing rod is hinged to the front end of the nail plate, and the upper end of the swing rod enables the front end of the nail plate to be in an upwards-lifted state. The flat nailing stapler has the beneficial effects of being high in nailing stability, good in flat nailing effect and the like.