The effect of minor Gd element on the microstructure and properties of Mg-8Zn-1Mn-3Sn alloy was studied. The results show that Mg-8Zn-1Mn-3Sn-xGd is mainly composed of α-Mg matrix, MgZn₂, MgZn, Mg₇Zn₃, Mg₂Sn phase and MgSnGd phase. The MgSnGd phase is a high temperature phase, which is formed firstly during the solidification and changes the solidification process, causing the semi-continuous second phase at the grain boundary to transform into a discontinuous network. The MgSnGd phase has a coherent orientation relationship with the α-Mg matrix, which can be used as a heterogeneous nucleation core to refine grains. The Mg-8Zn-1Mn-3Sn-0.5Gd alloy has the best comprehensive mechanical properties. The mechanical properties of the alloy are significantly improved by adding Gd elements to refine the grains, and the MgSnGd phase pins the grain boundary to hinder the movement of dislocations and the transformation of the second phase of the grain boundary.