Abstract: Compound excitation is an effective magnetized method, which can change electromag-netic coil current size so as to change the intensity of excitation according to the thickness of tank bottom plate. The method can hence meet the needs of different thickness of the workpiece detection and expand the scope of the test. In this paper, the compound excitation puts DC coil and permanent magnet as its excitation source. ANSYS simulation analysis is carried out on the compound excitation magnetic flux leakage testing to analyze the defect depth size, the number of ampere turns, which is the influencing factors of defect magnetic flux leakage signal. Simulation results show that with the increase of defect depth or the number of ampere turns, the intensity of magnetic flux leakage above the defects also increases.