A locking device includes: a pivot rod extending through a connecting block of a U-shaped seat and through a connecting block of a rotary cover such that the rotary cover is rotatable relative to the seat between unlocking and locking positions; a resilient member for biasing the rotary cover to move toward the locking position; and a position-limiting unit including a guiding groove formed in one of the connecting block and the pivot block, and a guiding rod provided on the other one of the connecting block and the pivot block, and extending into and engaging movably the guiding groove. When the guiding rod is limited and engaged in one end of the guiding groove, the rotary cover is positioned at the unlocking position, where the rotary cover is spaced apart from the seat.