Abstract: The T91 steel pipe of boiler superheater in a power station burst after serving for 79 583 h. The cause of the pipe burst was investigated through macromorphological observation, metallographic examination, energy spectrum analysis, chemical composition analysis, hardness test and fracture morphology observation. The results show that the T91 steel pipe burst due to the combined overheating caused by long-term overheating and short-term overheating. The generator units of the power station had long participated in the peak regulation and frequency modulation of the national grid, resulting in significant fluctuations in the unit load. This led to over-temperature operation, causing the aging of the steel pipe´s microstructure and the decrease of its mechanical properties. The alternating stress which was generated by the load fluctuations caused the oxide skin to fall off, resulting in local blockage in the pipeline and the sudden increase of the pressure in the pipe, triggering local over-temperature and finally causing the pipe burst. It was suggested that the heat load should be reasonably distributed to ensure the uniform distribution of fuel and avoid local overheating, and at the same time, the temperature distribution and material state of boiler pipes should be regularly checked to effectively prevent pipe burst.