Abstract: The 3D models of partial pipe, magnetization structure of the magnet yoke magnetization method and outer air are built using the finite element method. Through calculation, the effects of coil ampere turns, sectional area of iron core and the air gap of the contact surface to the distribution of magnetic induction field in pipe wall were quantitatively analyzed. When there were columnar defects and interstitial defects, the distribution of the magnetic leakage field was solved. The research results indicate that the more of the ampere turns, the more of the sectional area of iron core and the less of the air gap, the bigger of the magnetic flux density in pipe wall. There is a peak and change of direction in the magnetic induction field of the defects.