Abstract: Aiming at the problems of alloying element segregation and mechanical property degradation in PCTIG welded joints of Custom 455 maraging stainless steel, this study employed solution treatment as the pre-process, and systematic microstructural characterization and mechanical property tests were conducted on the joints treated at different aging temperatures(480 ℃, 520 ℃, 560 ℃), focusing on investigating the effect of aging temperature on the microstructure and mechanical properties of the joints. The results show that as the aging temperature increases from 480 ℃ to 560 ℃, the content of δ-ferrite in the joint gradually decreases, and its morphology gradually dissolves from vermicular to granular; meanwhile, the content of retained austenite increases progressively, with its morphology transforming sequentially from granular (480 ℃) to acicular (520 ℃) and then to continuous lamellar (560 ℃). The average hardness of the weld zone decreases from 526 HV to 400 HV, the tensile strength decreases from 1569 MPa to 1189 MPa, and the elongation increases from 6% to 12%. Mechanism analysis indicates that the reduction in δ-ferrite content can reduce the potential paths for crack propagation, thereby improving joint toughness; acicular retained austenite has a weak segmentation effect on the martensitic matrix, leading to a slight decrease in strength; continuous lamellar retained austenite forms a network structure, which significantly impairs the continuity of the martensitic matrix and results in a decrease in material strength. However, as a soft phase, continuous lamellar retained austenite can coordinate plastic deformation effectively and significantly enhance the continuous deformation capacity and fracture toughness of the material.