The invention relates to a method for manufacturing prefabricated gold-tin alloy on a copper-clad board in a 5G optical module. By using the characteristics of a dry film, the method perfectly achieves smooth surface lithography and a powerful pit-filling ability, is easy to operate, and greatly improves the yield of lithography. The ability to easily remove the dry film ensure a subsequent gold-tin alloy processing effect and efficiency, and provides convenience for mass production. Abandoning a traditional method of uniformizing photoresist or spraying photoresist and directly pressing the dry film against the surface of the copper-clad plate can ignore deep grooves on the surface, completely cover a product surface, and avoids worrying about unevenness of the surface caused by uniformizing photoresist or spraying photoresist. Then, gold-tin patterns can be lithographically produced on a copper surface with deep grooves by exposure and development.