Large sand casting magnesium alloys were prone to coarse grains,resulting in poor elongation at room temperature.In order to solve the above problems， the slow cooling conditions of large castings was simulated and the optimum process parameters of room temperature strengthening and toughening of large Mg-Gd-Y magnesium alloy castings were studied by composition optimization,microstructure analysis and heat treatment optimization.It is found that Mg-6Gd-3Y-0.5Zr (GW63K) has good comprehensive performance，the optimum solution treatment parameters for GW63K are 475 ℃/7 h+495 ℃/3 h,and the optimum aging treatment parameters are 200 ℃/80 h，and the average tensile,yield strength and elongation at room temperature are 334.5 MPa,201.0 MPa和6.2%,respectively.This study provides a reference for the application of Mg-Gd-Y alloys on large castings.