The invention discloses a method for protecting B15-grade fireproof completeness of a ceiling and a fireproof device. The method includes the following steps that S10, a fireproof cover is manufactured, specifically, the fireproof cover is made of B15-grade fireproof materials according to the actual sizes of an opening in the ceiling and a component installed at the opening in an embedded mode; and S20, the fireproof cover is installed, specifically, the fireproof cover is fixedly installed on the side, away from a cabin, of the ceiling, and the part, protruding out of the ceiling, of the component is covered by the fireproof cover. By arranging the fireproof cover with the B15-grade fireproofness and covering the component installed on the ceiling in the embedded mode with the fireproof cover, the fireproof cover can package the opening in the ceiling and the component arranged at the opening on the side, away from the cabin, of the ceiling, and the fireproof completeness of the ceiling is protected. When a fire disaster occurs, the ceiling and the fireproof cover form a complete fireproof layer, flame spreading is effectively prevented, and safety of a ship and personal safety of seamen are protected.