In order to reveal the effect of Zn on the microstructure and mechanical properties of Mg-Bi-Sn alloy, Mg-3Bi-5Sn-xZn (x= 0,1,2,3 wt.%) alloy was prepared by casting using OM, SEM, XRD, EPMA, Vickers hardness tester and tensile testing machine. The effect of Zn on the microstructure and mechanical properties of Mg-3Bi-5Sn (BT35) alloy was studied. The experimental results show that Zn can significantly refine the grain size of BT35 alloy. With the increase of Zn content, the grain size of BT35 alloy decreases significantly at first and then increases slightly, among which Mg-3Bi-5Sn-2Zn (BTZ352) alloy has the smallest grain size (58 μm). In addition, there are two kind of second phases, Mg₃Bi₂ and Mg₂Sn, observed in BT35 alloy, and additional Mg₂Zn phase and BiSn phase can be detected in the microstructure after adding Zn element. When the addition of Zn element is less than 3%, the tensile strength and elongation of the alloy firstly increase and then decrease slightly. Among them, BTZ352 alloy shows the best mechanical properties with the tensile strength and elongation of 263.5 ±6MPa and 13.2±0.6%, respectively; and the fracture microstructure of BTZ352 alloy exhibits typical transgranular fracture characteristics.