An impact device for an electric nail gun having a flywheel includes a swing arm unit and an impact unit. The impact unit includes an impact member adapted to be disposed between the flywheel and the swing arm unit, and a magnetic element set mounted to the impact member, spaced apart from the swing arm unit to form a gap therebetween, and providing a magnetic force which magnetically attracts a magnetically conductive component of the swing arm unit. The swing arm unit is movable between a pre-firing position where the impact member is not in contact with the flywheel, and a firing position where the impact member is in contact with the flywheel such that rotation of the flywheel drives the impact unit to move on the swing arm unit.