In this paper,the influence of the Sn element on the melt viscosity,grain size,shrinkage,and tensile properties of the sub-rapidly solidified Mg-6Al-4Zn alloy was studied.The results showed that the melt viscosity of the Mg-6Al-4Zn alloy was greatly decreased because of the addition of Sn.As the content of Sn increased from 0 to 1.8 wt.％,the grain size of the alloy was refined,and the dendrite microstructure was changed to rose-shaped ones simultaneously.The decreased melt viscosity and refined microstructure were conductive to the feeding of melt,which contributed to the reduction in volume fraction of shrinkage.The volume fraction of shrinkage of the Mg-6Al-4Zn-1.2Sn alloy was reduced by 30.8％,compared with that of the alloy without Sn addition.Tensile properties of the Mg-6Al-4Zn-xSn alloys were increased firstly and then decreased with the augmented Sn content.The yield strength,ultimate tensile strength,and elongation of the alloy containing 1.2 wt.％Sn were 21.4％,39.5％,and 259.0％ higher than those of the alloy without Sn addition,respectively.The addition of Sn was considered to reduce the shrinkage of the sub-rapidly solidified Mg-6Al-4Zn magnesium alloy and thus improved its tensile properties.To identify the mechanism,the effect of Sn on the volume fraction of shrinkage was discussed from three aspects of melt viscosity,grain refinement,and volume fraction of eutectic phases.