Stress concentration is an important cause of metal structure failures. In ferromagnetic materials, the nearby magnetic field changes when there is stress concentration. This phenomenon has been used to test stress concentrations. However, the mechanism that magnetic field changed by stress concentration is complicated, obtaining detailed stress information or even evaluating the impact of stress is difficult. This paper presents a method based on genetic algorithm to help analyze the stress distribution using the magnetic field. The ferromagnetic specimens are firstly discretized into finite elements and the initial stress value of elements is set as the “genes” of the initial parent chromosome. Then, the group “evolved”, and their descendants are selected with the law that the surface magnetic field shall close to the magnetic field. The results show that during the evolution, the stress distribution can be closer and closer to the real situation.