The additions of alloying elements can significantly improve the mechanical properties of magnesium (Mg) alloys, mainly due to the fact that their additions change the critical shear stresses (CRSS) for dislocation slips. In this work, experimental and computational methods were used to explore the mechanisms responsible for the roles of Sm element addition in affecting the mechanical properties of Mg. The results showed that the addition of Sm obviously improves the microstructure and mechanical properties. It promotes the formation of twins and beneficially activated the non-basal slip at the initial stage of plastic deformation, resulting in a high ductility. The Visco-Plastic Self-Consistent (VPSC) and the two-beam diffraction results confirmed that the pyramidal slip and prismatic slip were activated during tensile testing. The quantitative analysis of slip traces verified that the volume of non-basal slips reached 35 % after Sm addition. The additions of Sm with solid solution increased the activities of pyramidal dislocation during deformation, which was beneficial to accommodate the c-axis strain, and finally improved the room temperature ductility of Mg. First-principle calculations demonstrate that the solute Sm atoms would reduce the stacking fault energy for basal and prismatic slips.