A cover structure for a water bag inlet comprises: a base, a locking for mounting at the outer periphery of the base, and a cover for mounting on the base. An extension and a locking rim are respectively provided at two ends of the base. A groove is formed at the periphery of the base and is provided with locking blocks at an appropriate position. The locking ring is provided with locking sockets for combining with the locking blocks, and an opening at a pre-determined position of the inner rim of the locking ring, while a fastener is provided at respective ends and can be mounted with a cap, which has a switch at one end surface. Accordingly, the cover can be firmly combined with the base or freely detached to alter its assembly.