A connecting device for connecting a pellet chamber to a gearbox, includes an adapting disc and an annular adapter. The adapting disc has a front side fixedly connected with the pellet chamber, a rear side fixedly connected with a front side wall of the gearbox, and a through hole disposed at a middle part. The power of the gearbox is outputted from the through hole of the adapting disc. The annular adapter is laminated around the through hole of the adapting disc from the front side of the adapting disc, and fixedly connected with the front side wall of the gearbox. The adapting disc uniformly distributes the gravity of the pellet chamber and the impact force from the pellet chamber to the front side wall of the gearbox, thereby avoiding the gearbox from being broken due to impact.