Abstract: A tube burst accident of T23 steel platen superheater was found in a subcritical unit during shutdown and maintenance. The causes of tube burst were analyzed by means of macroscopic observation, metallographic examination, scanning electron microscopic analysis, mechanical property test and chemical composition analysis. The results show that the over-temperature operation of the unit resulted in serious decarbonization and over oxidation of the superheater tubes, which reduced the surface strength and fatigue resistance of the tubes. As a result, overload tensile fracture of the T23 steel platen superheater tubes occurred under high temperature environment, which led to tube burst.