Abstract: In view of the burst problem of Q235B heat-supply network pipe in a thermal power plant, the analysis by optical microscope, scanning electron microscope and tensile, impact properties test were carried out for the samples from different areas of the burst pipe.The results show that under the action of alternating load in the pipe, stress concentration occurs in the defect position with incomplete penetration, slag inclusion and porosity, cracks initiate and propagate, and eventually leads to fatigue fracture.