High Cr ferrite heat-resistant steel has excellent geometric structure stability, low radiation swelling rate, and good corrosion resistance of liquid metal. TP347H austenitic heat-resistant steel is based on the traditional 18-8 austenitic steel with the addition of a certain amount of Nb and a small amount of N to precipitate MX-type carbonitride, which results in superior high-temperature properties. Steam with high temperature and pressure flowing through supercritical thermal power units may exhibit heterogeneous connections between high Cr ferrite and austenitic heat-resistant steel components in the supercritical thermal power units. In this study, the vacuum diffusion-bonding of dissimilar materials between high Cr ferritic and TP347H austenitic heat-resistant steel was performed, the effects of diffusion-bonding time and post weld heat treatment (PWHT) process on the microstructural evolution and mechanical properties of the diffusion-affected zone was examined. The results indicated that with the extension of diffusion-bonding time, the interfacial bonding rate gradually increased. The interaction due to the difference in deformation storage energy and dislocation slips resulted in dynamic recrystallization, and the fine grains formed at the diffusion-bonding interface evolved into a serrated interface. Fine and dispersed MX and M₂₃C₆ phases were precipitated in the austenite grain boundaries and at the grain boundaries of the diffusion-bonding zone. After PWHT, the grains in the diffusion-bonding zone were further refined, dislocations were stable, dislocation density reduced, small-angle grain boundaries increased, and element diffusion was more sufficient. Tensile tests at different temperatures showed that the fractured sites were all in the matrix, which indicates that high-quality diffusion-bonding joints of dissimilar materials were achieved.