Abstract: Testing defect was found in a L458M steel pipeline during nondestructive testing. The conclusion of re inspection of the defect by X-ray and ultrasonic testing methods was quite different, and the type of defect can not be determined. The causes of the defect were analyzed by means of macroscopical analysis, metallographic analysis and energy spectrum analysis. The results show that the defect of the L485M steel pipeline was crack. Repair welding of pipe body has been carried out at this position. The crack was generated during welding and did not propagate during service. Copper pollution existed in the original damage location before repair welding. The crack was mainly caused by the liquid metal can not fill the shrinkage position in time during the solidification process of weld metal and the crystallization cracks caused by low melting point metal copper.