A lifting device for the backrest of a chair includes a fixed member, a movable member movably mounted on the fixed member and having an elongate slideway and a plurality of locking grooves, a locking bolt extending into the slideway of the movable member and detachably locked in one of the locking grooves of the movable member, a slide secured on the locking bolt to move the locking bolt, and an elastic member biased between the slide and the fixed member to push the locking bolt toward one of the locking grooves. Thus, the movable member is moved relative to the fixed member to adjust the height of the backrest so that the height of the backrest can be adjusted according to a user's requirement, and the backrest can support the user's back efficiently.