In view of the long-term service of large storage tanks, corrosion defects are easy to form on the surface of the tank bottom plate. In this paper, an adjustable MFL detection device was developed, which can adjust the detection height range of MFL from 1 to 20 mm, and defect detection tests were carried out with different ball hole defect sizes and different magnetized air gap spacing. The amplitude and voltage variation of MFL signal under different ball hole defect size and different magnetized air gap spacing is analyzed. The test results show that the amplitude voltage of MFL signal increases linearly with the increase of ball hole defect size radius. The amplitude voltage of the MFL signal decreases with the increase of the spacing of different magnetized air gaps. When the spacing increases to 12 mm, the ball hole defect of 20% specimen thickness is basically difficult to detect. By adjusting the lifting height of the MFL detection device, the defect location of the bottom plate can be effectively detected, and the corrosion detection problem of the tank bottom plate can be solved.