Zn20Sn solder with the melting point of 383.9°C and a low cost is considered as an ideal high-temperature lead-free solder. In the paper a new solder alloys were made by adding trace Cu into Zn20Sn alloy through alloying principle. Interfacial reaction and solderability of Zn20SnxCu (x=0 wt.%, 2 wt.%, 4 wt.% and 6 wt.%) solder on the copper substrate were investigated. Results showed that β’-CuZn, γ-Cu5Zn8 and ε-CuZn5 IMC layers were formed at the interface of Zn20SnxCu/Cu. The spreading areas of the Zn20SnxCu solders were reduced linearly with the increasing of the content of copper. The spreading aera of Zn20Sn solder was 52.88 mm2 while that of Zn20Sn6Cu was 50.82mm2 which was approximately 3.9% smaller than that of matrix solder. It is mainly related to the formation of ε-CuZn5 phase and the metal intermetallic compound between the solder and the substrate.