Abstract: In order to solve the problem of mechanical property deterioration caused by abnormal grain growth (AGG) of traditional Mg-RE-Zr alloy tungsten inert gas welding (TIG) repair welded joints during post-weld heat treatment, the effects of post-weld heat treatment on the microstructure and mechanical properties of Mg-4Y-3Nd-1.5Al alloy were investigated. The results reveal that after high-temperature solid solution treatment at 550 °C/24 h, the grain growth phenomenon in the weld zone of the alloy repair welded joint is effectively suppressed, with only a very small number of abnormally grown grains appearing, which is significantly better than that of traditional Mg-RE-Zr alloy welded joints. The thermal stability originates from the Zener pinning effect of numerous high-melting-point Al₂RE phases at grain boundaries. Calculations indicate that its pinning-limited grain size (about 97.8 μm) is much larger than the actual initial grain size of the weld. After solid solution (525 °C/24 h) + peak aging (200 °C/16 h) treatment, the tensile strength, yield strength, and elongation after fracture of the joint reach 293 MPa, 215 MPa, and 5.8%, respectively, and its mechanical properties are comparable to those of the base metal. Fracture analysis shows that the heat treatment effectively dissolves the brittle eutectic structure at grain boundaries, transforming the fracture mode from mainly intergranular to mainly transgranular. The study demonstrates that the Mg-Y-Nd-Al alloy, relying on the high thermal stability brought by the Al₂RE phase, can effectively coordinate the contradiction between eutectic dissolution and grain coarsening suppression during post-weld heat treatment, providing a new solution for the high-quality repair of magnesium alloy castings.