Abstract: Abnormal magnetic marks appeared in the magnetic particle testing process of a seamless steel pipe. The causes of abnormal magnetic marks were analyzed by hot acid corrosion test, metallographic examination, scanning electron microscopy and energy spectrum analysis, electron probe analysis and other methods. The results show that there was obvious banded composition segregation on the outer wall of the seamless steel tube. The composition segregation led to the improvement of austenite stability, the decrease of the initial temperature of martensitic transformation, the appearance of residual austenite in the microstructure, and the abnormal magnetic marks in the process of magnetic particle detection. These magnetic traces were distributed along the spiral direction, which had little effect on the longitudinal and transverse mechanical properties of the material.