The hydrogenation reactor is the core equipment of petrochemical hydroreforming, hydrorefining, hydrocracking and other units, which operates over long term under harsh conditions, e.g., high temperature, high pressure, hydrogen. In China, a considerable number of hydrogenation reactors have been in service for more than 20 years, and the longest service has reached 48 years. How to ensure the safe operation and reasonably extend the service life of long-term service hydrogenation reactor is an urgent engineering and technical problem. In this paper, the theoretical exploration and empirical research progress on safety guarantee of in-service hot-wall hydrogenation reactors in the past 40 years are reviewed, including failure modes, degradation mechanism of material performance, inspection and maintenance methods, safety assessment and remaining life prediction technologies, welding repair technologies, and improvement of operation process, etc. In view of the main problems existing in the safety guarantee technology of hydrogenation reactors which have been in service for more than 30 years, suggestions for further research are put forward, including establishment of a database for long-term service hydrogenation reactors (20∼50 years), development of high-precision non-destructive testing technology, research on the occurrence, development and termination laws of the potential failure mechanism of long-term service hydrogenation reactors, optimization of safety assessment technology, research on the ultimate service life assessment technology, in order to promote the technical progress of safety guarantee of hydrogenation reactors which have been in service for more than 30 years.