Abstract: Penetrant test, metallographic examination, fracture analysis and mechanical properties test were used to analyze the causes of girth weld cracking in high temperature section of boiler feed water preheater tube box. The results show that the post-weld heat treatment did not meet the requirements, and martensite was formed in the girth weld and heat affected zone. During the service of the equipment, martensite decomposed to produce precipitated phase, which reduced the creep ability of the grain boundary. Under the high temperature operation environment, the girth weld produced reheat cracks and intergranular cracks.