Abstract: In the petroleum and petrochemical industry, tank bottoms are often corroded and patch repairs are performed. Accurate evaluation of the bottom corrosion of the tank floor with patch plates is an urgent task for the inspectors. In this paper, the magnetic flux leakage detection technology is used to study and analyze the tank bottom plate and the patch plate. The finite element model of the leakage magnetic field of the patch defect is established, and the spatial distribution characteristics of the leakage magnetic field at different positions of the defect are obtained. Experiments were carried out on different positions of defects under laboratory conditions. The experimental results were consistent with the simulation analysis. The magnetization structure magnetizes the plate to a near-saturation state during the magnetization of the part to be inspected. The degree of magnetization and the degree of magnetization of the single plate experiment are similar. However, the leakage magnetic field generated at the patch defect will preferentially pass through the original substrate that has not been magnetized to saturation, resulting in a defect quantification result that is smaller than the actual value.