Abstract: There was an accident happened in a steel and iron company that the water-cooling tube of power station boiler broke,while the boiler only worked for about 15,000 hours. After analyzing the chemical composition and metallographic pattern of the tube and the X-ray diffraction analyzing of the scale structure, the result showed that the scale inside the tube blocked the heat conduction and caused the tube broke partly. The scale appeared because the boiler worked with the water which had not been softened.