Nondestructive testing for ferromagnetic material equipment based on low-frequency electromagnetic technique (LFET) and signal difference is proposed. Magnetic field distributions of the defect area in or on the 20# steel surfaces are analyzed using the finite element method (FEM) based on COMSOL Multiphysics. After the signal difference converting, for a surface defect in the 1.2 to 9.6 mm depth, amplitude voltage ranges of 1.8 to 4.2 V and 2.5 to 3.5 V are obtained for the defects in 20# steel plate and pipe, respectively. In addition, compared with the traditional MFL, the 4.8-mm buried internal depth and the 9.6-mm depth surface defect are detected in the 12-mm-thick 20# steel plate or pipe, which is about 1.6 times higher than that of the current similar sensors.