Abstract: There were many cracks in the weld and heat affected zone of the butt joint of the high pressure steam pipe of a 12Cr1MoVG steel boiler. The cracking causes were analyzed by means of macroscopic observation, chemical composition analysis, mechanical property test, hardness test, metallographic examination and scanning electron microscope analysis. The results show that the cracking of butt joint was caused by the combined action of unqualified welding wire material, improper control of welding process parameters, long-term operation of pipeline in reheat crack sensitive temperature range and thermal stress.