AuSn20 eutectic solder is an important material for high-power and high-reliability electronic packaging due to its superior mechanical and thermal conductive properties. In this paper, the AuSn20 eutectic film for high accuracy assembly is preformed precisely on AlN substrate via the stack electroplating method. The stack electroplating is cost-effective to improve the pattern accuracy when compared to traditional powdered metallurgy methods. The AuSn20 eutectic solder is achieved through the following processes: (1) cleaning the AlN substrate; (2) sputtering TiW/Ni/Au seed layer on AlN substrate; (3) photolithography pattern; (4) electroplating Au on the seed layer; (5) electroplating Sn on the Au layer; (6) electroplating Au on the Sn layer, forming the Au/Sn/Au stack film which meets the ratio of AuSn20 solder; (7) alloying Au/Sn/Au stack film. The AuSn20 eutectic film is analyzed to be Au5Sn phase and AuSn phase, its shear strength meets the relevant requirements of GJB548B-2005, method 2019 die shear strength. The laser diode is attached on AlN substrate by the AuSn20 eutectic film, which has good consistency and performance, and the optical power efficiency reaches 35%.