Abstract: During 168 h full-load trial operation of an ultra-supercritical unit, cracking occurred at welded joints of 12Cr1MoV steel low re-export header socket and connecting pipe. The cracking cause was analyzed by macroscopic morphology observation, chemical composition analysis, mechanical property test and microstructure observation. The results show that the service temperature of the welded joint of 12Cr1MoV steel socket and connecting pipe was in the reheating crack sensitive temperature range. the reheating cracks were initiated quickly and expanded in the wide heat affected zone under the superposition of structural stress, constraint stress generated by forced welding and welding stress which was not eliminated by heat treatment, and eventually led to cracking of the joint. It was recommended to adopt a same-diameter and equal-wall-thickness connection method between 12Cr1MoV steel connecting pipe and socket. The strong matching was strictly prohibited in the welding process. Low welding heat input should be adopted and post-weld heat treatment should be carried out to reduce the sensitivity of reheating cracking.