Abstract: During the operation of a 300 MW unit boiler in a power plant, it was found that the 12Cr1MoVG steel plate reheater burst. By means of macroscopic observation, chemical composition analysis, mechanical properties test, hardness test, metallographic examination, scanning electron and energy spectrum analysis, the causes of tube explosion were analyzed. The results show that the crater belong to long-time over-temperature cracking, and the microstructure at the crater was seriously aged (nodulizing grade reached five). There were many longitudinal creep cracks in the inner and outer walls around the detonation, the inner and outer walls of the blast hole were covered with a thick oxide layer. Oxidation and coking on the outer wall led to the local long-time over-temperature of the elbow. The microstructure and properties of the pipe at the overheating point were seriously degraded, which made the local creep and produced creep cracks. The cracks propagated along the crystal, and finally creep cracking occured under the pressure of the internal medium.