A copper-tin brazing wire and a preparation method and use thereof are provided. A copper-tin brazing wire includes a plurality of copper wires each having a composite metal layer on a surface thereof; the copper-tin brazing wire includes, in parts by weight, 75-84 parts of Cu, 20-25 parts of Sn, and 0.4-0.5 parts of P; and the composite metal layer includes Cu, Sn, and P, in which a mass ratio of Cu, Sn, and P is (45-55):(46-56):(0.5-1.5).