The invention discloses a preparation method of a copper aluminum composite board, and the method comprises the following steps of: 1) carrying put single-face oil removal and scale removal on copper board strips, and coating a layer of brazing flux, for later use; and 2) oppositely setting the surfaces of the two copper board strips coated with the brazing flux, and injecting semisolid aluminum between the two copper board strips to carry out oxygen-free continuous casting and rolling, so as to obtain the copper aluminum composite board. The preparation method of the copper aluminum composite board comprises the following steps of: preparing the copper aluminum composite board by an oxygen-free continuous casting and rolling method, cleaning the surfaces of the copper strips and coating a layer of the brazing flux before the process of continuously casting and rolling, wherein the liquid aluminum liquid is equal to brazing filler metal in the process of continuously casting and rolling; and carrying out welding after the aluminum liquid is contacted with the copper board strips, wherein the brazing flux coated on the surfaces of the copper board strips can be used for removing residual oxide on the surface of the brazing filler metal (the aluminum liquid) and the copper board strips, improving the wettability between the aluminum liquid and the copper board strips, and improving the seaming quality, so that the bond strength of the copper aluminum composite board can be greatly improved and can reach about 150MPa.