A structure of the lockable door includes a door cover, a first sliding member, a fastening member, and a second sliding member. The door cover is hinged to a case, and can selectively cover an opening of the case. The first sliding member is inserted through the door cover, and can be slidably mounted on one side surface of the door cover. The fastening member is slidably mounted on the other side surface of the door cover, and has at least one protrusion. The first sliding member is inserted through the fastening member, and the first sliding member and the second sliding member are connected to each other. When the first sliding member is in a retaining position, the second sliding member moves to be on the protrusion, and the first sliding member presses the door cover towards the second sliding member, so as to enhance waterproofness.