A cap locking device for a water bottle comprising a bottle cap threadably closing an inner bottle, a push button and a nipple pusher, the bottle cap shaped like a shell head having a diametric opening, a tubular post in the opening for a nipple fixed on top of a drinking tube extending vertically in the inner bottle to fit therein, a push button pivotally fitted in one side portion of the opening and the liftable nipple pusher pivotally fitted in another side portion of the opening, the liftable nipple pusher pushed down to close the opening or raised up to open the opening by pushing the push button for the nipple exposed for sucking the content of the inner bottle contained in an outer bottle.