Abstract: Tube burst occurred in platen superheater of an ultra-supercritical unit. The cause of tube burst was studied by means of macroscopic observation, chemical composition analysis, hardness test, metallographic examination, scanning electron microscope and energy spectrum analysis. The results show that the microstructure near the explosion port was aged, and the large or long σ phase was precipitated at the grain boundary. The closer to the explosion port, the more σ phase, the larger the grain size, and there were holes and microcracks around the σ phase, which eventually led to the tube explosion.