Abstract: The compound excitation method is a method that combines permanent magnet and DC magnet excitation method. ANSYS finite element software is used to simulate the process defect leak and achieve the spatial distribution of the magnetic field. Using compound excitation magnetic flux leakage detector which was made of good size effect of magnetization, cascade compound excitation magnetic flux leakage testing experiments were finished for Q235 steel which was of 6, 8, 10mm, twine 600 turn coil winding, inlet the electric current I=0, 2, 4, 6, 9A in turn. The experimental results show that when dc current is increased, the ability of magnetization is enhanced and the detection effect is better. Meanwhile a fitting curve of peak leakage magnetic field of the defect with different currents is obtained.