Abstract: During the inspection of a hydrocarbon mixing spherical tank, a large number of surface cracks were found at the lower plate girth weld position. The residual stress at the crack concentration position was tested by X-ray stress tester, and the cause of cracking of the spherical tank was analyzed by chemical composition analysis, mechanical property test and fracture micromorphology observation. The results show that the sulfide stress corrosion cracking occurred at the fusion line and heat affected zone in the lower plate girth weld of the spherical tank. Under the action of corrosive medium and large residual tension stress, the cracks initiated in the heat affected zone of the lower plate girth weld with the largest residual tension stress, and then propagated to the fusion line in the transgranular and intergranular forms. Reasonably selecting materials before welding, selecting suitable welding and heat treatment technology and improving the quality of stress relief heat treatment after spherical tank installation should be taken to reduce the residual stress; isolating corrosive media and removing hydrogen sulfide or water from the media were used to reduce the sensitivity of cracking.