To reveal the effect of Zn element on the microstructure and mechanical properties of Mg-Bi-Sn alloy, Mg-3Bi-5Sn-xZn (x=0, 1, 2, 3, wt%) alloys were prepared by casting. Using OM, SEM, XRD, EPMA, Vickers hardness tester and tensile testing machine, the effect of Zn element on the microstructure and mechanical properties of Mg-3Bi-5Sn (BT35) alloy was studied. The experimental results show that Zn element can significantly reduce the grain sizes of BT35 alloy. With the increase in Zn content, the grain size of BT35 alloys decreases significantly at first and then increases slightly, among which Mg-3Bi-5Sn-2Zn (BTZ352) alloy has the smallest grain size (58.2 μm). In addition, there are two kinds of second phases, Mg₃Bi₂ and Mg₂Sn, observed in BT35 alloys, 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 3wt%, 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±6.0 MPa and 13.2%±0.6%, respectively; and the fracture microstructure of BTZ352 alloy exhibits typical transgranular fracture characteristics.