Space equipment represented by launch vehicle requires lightweight materials of heavy components to meet the demanding weight reduction. Magnesium alloys as the lightest industrial structural metal material is regarded as the preferred materials of lightweight, and has been applied preliminarily in defence equipment. However, the essential high-quality large-size ingot, which is the raw materials of high-performance wrought magnesium alloy components with the ultimate tensile strength greater than 400MPa, exists microstructure uniformity and properties weakening in larger objects due to size effect. It leads to limited application of wrought magnesium alloy. The newly metal additive forging technology proposes a “small to large” approach to manufacture heavy homogeneous ingot by using the metal base elements with small-size. In this study, the metal additive forging was firstly applied to the preparation of high-performance Mg-Gd-Y magnesium alloy forgings. After surface cleaning, stacking, diffusion welding and multidirectional forging, the ultimate tensile strength increased from 245 to 273MPa and elongation to failure increased from 3% to 8.1% compared with solution heat treatment condition. After three passes forging, the thickness of oxide layer decreases with the closed healing area enhancing in the interface. The ultimate tensile strength, tensile yield strength and elongation to failure of the forging samples after ageing, which is 400 MPa, 248 MPa and 8.4%, respectively. It confirmed the feasibility of fabricating large-size magnesium alloy by metal additive forging method. This paper provides technical supports to manufacture high-performance metal heavy components for industrial production.