The mechanism of metal magnetic memory on ferromagnetic materials under loading and geomagnetic field was studied.Based on the principle of magnetic memory,the ferromagnetic materials with characteristics of spontaneous magnetization and magnetic domain,as well as the magnetic mechanism,we propose a new physical mechanism model of magnetic memory testing.Based on this model,we can conclude that the tested magnetic field on the surface is the result of vector superposition of the leakage magnetic field generated by defects,the inductive magnetic field of the specimen itself,and the environmental magnetic field.The magnetic field of a rectangular slot was simulated by matlab.The phenomena in the course of magnetic memory testing can be explained well.