A nailing unit includes a nail box, a nail magazine, a nail pushing block, a tail buckle switch, a nail limiting block fit together with a nail guiding plate through slits is provided. The nail magazine includes a heat-treated guide rail assembly mounted with a nail splitting plate at the end near the nail guiding plate. The nail pushing block connected to a spring is formed with three nail pushing feet adapted to a first, second, and third gaps. The nail guiding plate includes two connecting wings and forms a low-lying guide plate therebetween. Each angle between the guide plate and each connecting wing is respectively fixedly provided with a positioning step along the longitudinal direction.