The metal magnetic memory (MMM) method can be used to predict and evaluate the microdamage region of ferromagnetic metal components in early stage; however, the magnetic memory signal is easily disturbed by external strong magnetic field, which can lead to the deviation of the detection result. In order to study the influence rule of magnetic field intensity on magnetic memory signal, the Kp perturbation algorithm is used in this paper, the multiprimary cell magnetic mechanical model is established based on the number of effective Bohr magnetons p in the K space, and the quantitative changing relationship of magnetic mechanics under the action of external magnetic field is calculated. The research results show that under the action of external magnetic field, the electron orbit motion is strengthened, the crystal lattice structure is distorted, and the magnetic moment of the atom is increased. When the magnetic field is small, the magnetic memory signal increases with the increasing of the external magnetic field intensity linearly; When the magnetic field intensity reaches a critical value, the magnetic moment of the atom is approximately equal to the magnetic moment of the independent atom, and the magnetic memory signal tends to be a constant value; When the magnetic field intensity is greater than the critical value, the magnetic memory signal of the stress concentration area will be covered by the magnetic field. The experiment results of the magnetic memory testing obtained in this paper and theoretical analysis results have very good consistence.