To explore the influences of plastic stress and residual stress on the magnetization of ferromagnetic steel in geomagnetic field, series of experiments and analyses are conducted in this paper. Two different types of specimens made of Q195 carbon steel with/without initial geometric discontinuities are manufactured. Various plastic tensile stresses are introduced into the specimens and the corresponding applied stresses, deformation distributions and residual magnetic fields of specimens are measured. The gradients of the residual magnetic field of the specimens are computed and the relationship between the residual magnetic field and the plastic loading stress is summarized. It is concluded that the intensity of magnetization of ferromagnetic steel in geomagnetic field increases with the plastic stress, but scattered by the dislocation. The influences of residual stress on the magnetization of ferromagnetic steel are not significant in the geomagnetic field.