It is difficult for traditional magnetic flux leakage methods to detect inner surface defects of thick-walled steel pipe or plate due to magnetic shielding of the wall and strong magnetic background noise, and for eddy current testing as well due to its skin effect. Hence, the “perturbation” course of internal magnetic field variations is analyzed using magnetic dipole model and the mechanism of magnetic permeability perturbation in MB-ECT is revealed. The theoretical analysis and simulations show that the significant permeability perturbation always appears around the defect. A new nondestructive testing method based on permeability perturbation is proposed for the detection of corrosion in thick-walled pipe.