Abstract: The welded joint of 15CrMoR steel desulfurization tank in a synthetic ammonia methanation unit was cracked, leading to leakage after operation for 10⁵ h. The reason for cracking of the joint was analyzed by macromorphology observation, chemical composition testing, mechanical property testing, microstructure observation and fracture analysis. The results show that hydrogen embrittlement was the main reason for cracking failure of the desulfurization tank joint. Comparing with the base metal, the weld and heat-affected zone had higher strength and higher hydrogen embrittlement sensitivity. After long-term serving in a high temperature and high pressure hydrogen environment, hydrogen atoms gathered in the weld and heat-affected zone on the inner surface of the desulfurization tank, resulting in hydrogen-induced cracking. Because of frequent internal leakage of the heat exchanger in the methanation device, the desulfurization tank experienced multiple emergency shutdown, resulting in accumulation of damage of the welded joint, which accelerated crack propagation, and led to cracking and leakage of the desulfurization tank.