Three potassium fluorozirconate (K2ZrF6) salts mixture (M) were prepared and added to the as-cast Mg-3Y-3.5Sm-2Zn alloys at three different temperatures. The microstructures and mechanical properties of the alloys were investigated by differential thermal analysis (DSC), X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and tensile test. The grain refinement mechanism of K2ZrF6 salts mixture was also discussed. The results show that the as-cast Mg-3Y-3.5Sm-2Zn alloy is composed from three phases, namely α-Mg, Mg12(Y,Sm)Zn and (Mg,Zn)3(Y,Sm). The addition of K2ZrF6 salts mixture does not lead to the formation of any new phase detectable by SEM and XRD. M3 salt (60wt% K2ZrF6-20wt% NaCl-20wt% KCl) added at 780°C exhibits the best grain refining effect. The mechanical properties of the as-cast Mg-3Y-3.5Sm-2Zn alloy are improved by three salts added at 780°C, where the alloy refined by M3 salt has the best mechanical properties. The grain refinement mechanism of K2ZrF6 salts mixture in the as-cast Mg-3Y-3.5Sm-2Zn alloy is attributed to the grain refining effect of fine Zr particles and zirconium-rich zones obtained by the reduction reaction between Mg and K2ZrF6.