Abstract: The plastic deformation of oil and gas pipeline was tested by magnetic memory testing. The energy balance under plastic deformation of ferromagnetic material was analyzed through dislocation theory. The change characteristic of magnetic signal was analyzed by ferromagnetic material energy balance theory. The bulge test was applied to pipeline. The magnetic signal was tested by magnetic memory testing equipment. The correspondence between magnetic memory signal and stress was studied. The change characteristic of magnetic signal along with stress change was obtained. The results show that surface weak magnetic signal will sudden change when plastic deformation emerges in ferromagnetic material. Moreover, the slope of stress-magnetic field curve rises. The repeated plastic deformation shall reduce magnetic field change rate.