Abstract: On the question of unsaturated magnetic field of the oil and gas pipeline being liable to resultin undetected defects, high magnetic permeability of the ferromagnetic material was used to develop the MFL test instrument. This article introduced the principle of the detection and the various parts of the device comprehensively. The ANSYS 3D model was constructed and the critical value of the non-saturation magnetic field was found. Through massive physical experiments, the electromagnetic correspondence relationship between the detection signals and the steel sheet was analyzed. The test results showed that the apparatus could detect the defects of 10% of the upper and lower surfaces of the steel plate with less than 20mm thickness, under the appropriate magnetization condition.