In recent years, advanced ultra-supercritical (A-USC) power plants have developed rapidly to increase the thermal efficiency and decrease CO₂ emission. Inconel 740H (IN 740H) is one of the Ni-based superalloys with the highest creep strength and good corrosion resistance at elevated temperatures. Owing to its excellent comprehensive properties, IN 740H is considered one of the best candidate materials for superheater and reheater in A-USC power plants. Further improving the mechanical properties of IN 740H welded joint enhances the safety and economic viability of the power plants. In this study, IN 740H tubes were welded by multipass tungsten inert gas hot-wire welding followed by a post weld heat treatment (PWHT) at 800 ^oC for 5 h. The formation mechanism of the γ′-denuded zone in the IN 740H welded joint after creep at different temperatures was systematically investigated using OM, SEM, and TEM, and its effect on the mechanical properties was analyzed.Results show that the small rod-like γ′ phase only discontinuously precipitates at grain boundaries in the weld metal during PWHT. After creep at different temperatures, an earlier formation of a coarse rod-like γ′ phase and γ′-denuded zone is observed at grain boundaries in the weld metal than in the base metal. The formation of the coarse rod-like γ′ phase at grain boundaries in the base metal results from the discontinuous coarsening of the spherical γ′ phase near grain boundaries, whereas that in the weld metal results from the discontinuous coarsening of the discontinuously precipitated rod-like γ′ phase at the grain boundaries and spherical γ′ phase near the grain boundaries. The discontinuous coarsening of the rod-like γ′ phase and precipitation of M₂₃C₆ carbides at grain boundaries lead to the formation of the γ′-denuded zone. Increasing the creep temperature and creep time when the temperature is in the range of 700-800 ^oC increases the size of the rod-like γ′ phase and width of the γ′-denuded zone at grain boundaries, whereas the number of rod-like γ′ phase initially decreases and then increases with the increase of creep temperature. The spherical γ′ phase in the grain interiors plays a vital role in changing the hardness of the IN 740H welded joint. The discontinuous coarsening of the γ′ phase and the formation of the γ′-denuded zone at the grain boundaries not only decrease the hardness, but also deteriorate the creep rupture strength of the IN 740H welded joint. Controlling the discontinuous coarsening of the rod-like γ′ phase at grain boundaries, suppressing the formation of the γ′-denuded zone, and controlling the growth of the spherical γ′ phase in the grain interiors are necessary to improve the mechanical properties of the IN 740H welded joint.