An engaging device of a linear sliding rail includes a sliding rail and a sliding block. The sliding block comprises a sliding base body, a supporting body and two connecting bodies. The front and rear ends of the sliding base body have a positioning plate respectively. The two locking pieces each comprises hooks. The hook locks the two connecting bodies at the front and rear ends of the sliding base body. The hook abuts against one end of the abutting piece. The other end of the abutting piece abuts against front and rear end faces of the supporting body respectively. Via this arrangement, when the locking piece locks the two connecting bodies at both ends of the sliding base body, it also generating a pre-pressure to press the supporting body, thereby assembling the supporting body and the two connecting bodies on the sliding base body tightly and firmly.