A flywheel device is adapted to be installed in an electric nail gun. The electric nail gun includes a main frame, a drive pulley rotatably mounted to the main frame, and a transmission belt. The flywheel device includes a flywheel pulley adapted to be mounted to the main frame, a flywheel connected coaxially and co-rotatably to the flywheel pulley, and a weight disc connected coaxially and co-rotatably to the flywheel and the flywheel pulley. The transmission belt is trained on the drive pulley and the flywheel pulley. During co-rotation of the flywheel pulley, the flywheel and the weight disc, a swing arm is pivotable to move an impact member to contact the flywheel for actuating a nailing process of the electric nail gun.