Abstract: The perforation leakage of an in-service pipeline during pressure test was studied by means of macro observation, chemical composition analysis, mechanical property test, metallographic examination and scanning electron microscope analysis. The results show that the main reason for the perforation of steel pipe was high-temperature burn on the external surface of the pipe, and the external surface of the pipe had been burned and penetrated before the pressure test, and then leakage occured during the pressure test.