The invention provides a casing combination, which comprises a plurality of branch casings. Each branch casing comrpsies a thick tube body, a thin tube body and a first stop part.The thick tube body is provided with an inlet. The thin tube body is provided with an outlet communicated with the thick tube body. The inelt and the outlet are respectively positioned at the two ends of each branch casing. The first stop part is positioned on the inner wall of the thick tube body. A collection region is arranged between the thin tube body and the first stop part. The thin tube body of each branch casing can be inserted into the inlet of another branch casing so as to be mutually combined together. The thin tube body of each branch casing can partially pass through the first stop part of another branch casing, so that the outlet of each branch casing is positioned within the collection region of another branch casing.