The invention discloses a copper/aluminum composite strip and a preparation method thereof and belongs to the technical field of metal material machining. A surface layer of the composite strip is a copper layer, an aluminum-silicon alloy layer is arranged in the middle, a bottom layer is an aluminum layer, and the ratio of thicknesses of all the layers is 1:0.02:2; the composite strip comprises chemical components in percentage by weight as follows: 62.00% of copper, 0.05% of silicon and 37.95% of aluminum. The preparation method of the composite strip comprises steps as follows: firstly, original metal strips, namely, a copper strip, an aluminum-silicon alloy strip and an aluminum strip, are annealed, then the annealed strips sequentially pass through an uncoiling machine, a straightening machine, a polishing device, a photoelectric alignment device and a composite rolling mill for composite rolling for the first time, and finally, 8-pass finish rolling is performed. According to the copper/aluminum composite strip and the preparation method thereof, when the preparation process is compared with preparation processes of common copper/aluminum composite strips, 66.7% of a copper material in percentage by volume can be saved; one aluminum-silicon alloy layer is added in the middle, so that the copper strip and the aluminum strip are effectively prevented from directly acting to form an intermetallic compound; besides, the copper/aluminum composite strip is higher in plasticity.