A large number of cracks are found in the 1500m3Liquefied Light hydrocarbons vessel during the inspection after two years of running.In totally,six vessels being fully heated treated is made of 15MnNbR. Researchers used X-ray stress analyzer measured the welding residual stress of vessels that have been cracked and uncracked. The results show that, residual stress of uncracked spherical vessels are in good condition, while those value of cracked is high, the value of the highest is 1.05σs. It indicates that the possible reason for the separation is the residual stress in the high state. Through welding and heat treated for the second time, the welding residual stress decreased obviously. It indicates the impact of this measure is good.