Achieving carbon neutrality requires advanced hydrogen-based processes, such as direct reduction of iron ore, wherein the long-term stability of structural materials under high-temperature and high-pressure hydrogen is critical. This study directly visualizes hydrogen-induced microstructural evolution in Inconel 600, a Ni-based alloy, under 1-3 GPa employing in situ high-energy X-ray diffraction, transmission electron microscopy, and electron backscatter diffraction. Hydrogen absorption promoted pressure-dependent lattice expansion, hydride formation behavior, and dislocation multiplication. At 400 °C, fine cracks formed along grain boundaries and intersecting slip bands, whereas at 800 °C, abundant intragranular voids were observed. The migration of planar dislocation generated slip bands that served as preferential pathways for crack propagation, facilitating hydrogen reaction embrittlement. Minor lattice contraction following isothermal holding reflected hydrogen desorption and carbide gasification (CH₄ formation from Cr₇C₃-type carbides). These findings reveal a sequential, chain-like evolution of microstructure under extreme hydrogen environments, providing mechanistic insights for the design of durable Ni-based alloys for hydrogen-utilizing carbon-neutral technologies.