A warehouse management system for automatically weighing and counting parts includes a management computer and a weighing device. The management computer communicates with the weighing device. The weighing device includes a base, a tray for carrying a parts bin, and a weighing module. A track is mounted to the base. A roller is mounted on the bottom of the tray and rides on the corresponding track. The tray can move over the track by the roller. The weighing module is attached to one end of the track. The tray together with the parts bin can be actuated to move to the weighing module so as to weigh and count the parts within the parts bin.