Abstract: As a clean energy source, hydrogen is widely regarded as an effective strategy for replacing fossil fuels and mitigating environmental issues. Based on the domestic and international research on the occurrence state and phase of hydrogen, and combined with the variations in temperature and pressure under geological conditions, the authors concluded the following insights. ① Natural hydrogen is found in free, dissolved, and adsorbed states in earth's shallow layers and in metal hydrides state in earth's deeper layers. The transformation among these states constitutes a pivotal factor influencing the availability of hydrogen resources. ② In geological settings, natural hydrogen predominantly exists in a supercritical state, and it only transitions to a gaseous form near the surface. Further research is warranted to investigate its characteristics. ③ The potential for natural hydrogen development remains largely unrealized. Future research should focus on the accumulation patterns, migration mechanisms, and preservation conditions of hydrogen to thoroughly explore its development potential. Additionally, the transformation processes between different states provides a novel perspective on understanding the earth's hydrogen cycle. The study on the occurrence state and transformation processes of natural hydrogen could provide theory basis for the accumulation patterns of natural hydrogen, and references for further development and exploitation of natural hydrogen.