Stress concentration is a major cause of metal structure failures. In ferromagnetic materials, it changes the nearby magnetic field. This phenomenon has been used to test stress concentrations. However, magnetic field changes caused by stress concentrations are complicated; therefore obtaining detailed stress information from magnetic fields or even evaluating the impact of stress is difficult. This paper presents a simulation of magnetic field changes caused by stress concentrations. The simulation discretizes ferromagnetic specimens into finite elements and assumes stress and magnetization evenly distribute in every element. Then, the magnetization change of every element caused by stress concentration is calculated by the theoretical model of magneto-mechanical effect. Finally, the magnetic effects of elements are merged together to simulate the external magnetic field. The results agree with the experiments on specimens with the same configuration in a finely controlled magnetization and stress environment. This simulation is meaningful for acquiring more quantitative information about stress concentrations from magnetic fields.