The microstructure and mechanical properties of the ZM61 alloy (Mg-6Zn-1Mn) with different Sn content were researched by optical microscope (OM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and tensile tests. The existence form and acting mechanism of Sn element in ZM61 alloy and the influence of Sn content on microstructure and mechanical properties were investigated. The results show that the addition of Sn can improve the casting fluidity and the formation of the dispersed Mg2Sn particles can refine the MgZn phase to improve the microstructure and the mechanical properties of ZM61 alloy. The ZM61-4Sn alloy with double aging treatment has the best mechanical properties, with tensile strength of 374 MPa, yield strength of 358 MPa and elongation of 4.6%. However the addition of Sn cannot change the fracture behavior of ZM61 alloy while the coarse Mg2Sn phase can become crack source during fracture to decrease the plasticity. Therefore, Sn content should not be too high, no more than 4%.