After homogenization treatment, uniaxial experiments at room temperature with strain rate of 1 s⁻¹ are carried out on Mg-Gd-Y(-Sn)-Zr alloys, and the maximum strain is controlled to 8%. Plenty of {10–12} tensile twins appear in the deformed microstructure of Mg-Gd-Y-Zr and Mg-Gd-Y-Sn-Zr alloys. The addition of Sn significantly improves the nucleation ability of twins and effectively reduces the lamellar thickness. In Mg-Gd-Y-Zr alloy, the twins are mostly coarse parallel, while in Mg-Gd-Y-Sn-Zr alloy, they are mostly thin cross. A great quantity of closed regions and twin boundaries introduced by high-density thin cross twins are the main reasons for the high flow stress of Mg-Gd-Y-Sn-Zr alloy. In addition, the analysis of twin variant selection behavior, twin interaction and dislocation transmutation reaction in this paper is conducive to consummate the theory of room temperature deformation of magnesium alloys.