Abstract: The T91 steel supporting tube of primary superheater of ultra supercritical boiler in a power plant leaked after operating for 1 400 h. By means of macroscopic examination, chemical composition analysis, mechanical property test, hardness test and metallographic examination, the causes of leakage of supporting tube were analyzed. The results show that the pressure strength of the supporting tube decreased sharply due to its short-time overheating. Under the internal medium pressure, the tube section expanded and the wall thickness decreased. When the wall thickness was not enough to resist the internal pressure, the tube section bursted, resulted in the leakage of the supporting tube.