Effects of different Sn additions on microstructure and mechanical properties of free-cutting Bi brass were investigated by Philips-XL30 SEM （scanning electron microscope） and HY-1080 electron tensile testing machine, and the cutting performance of the alloy was examined by C6136 horizontal lathe. The results show that Sn can effectively improve the property of the alloy by changing the distribution of Bi distribution to make Bi to move from the phase boundary into the α phase. However, when Sn content exceeds 2%, CuZnSn phase , a brittle electron compound, can be created in the alloy, resulting in the deterioration of mechanical properties. The cutting performance of the alloy is improved with increasing in Sn content.