Abstract: The 12Cr1MoV steel steam pipe of a boiler was cracked during operation. The cracking causes of steam pipe were analyzed by means of macroscopic analysis, chemical composition analysis, microscopic analysis, metallographic examination and mechanical property test. The results show that there was a certain degree of stress concentration in the processing trace of inner wall of the steam pipe. The oxidation corrosion occurred in the high temperature environment, which increased the stress sensitivity. When the steam pipe was working, it was subjected to the thermal stress caused by the temperature change and the alternating stress caused by the vibration of the steam passing through. Under these two forces, the corrosion pits produced stress concentration and then formed microcracks. The microcracks propagated under the action of high temperature and alternating stress, and finally penetrated the pipe wall.