The magnetic-thermal-flow field coupling in the process of laser additive manufacturing of Ni45 alloys under alternative magnetic fields is numerically investigated by using the finite element and finite volume methods. The influence of the external magnetic field on the evolution trend and properties of solidification structures is investigated based on experiments. The results show that the introduction of external magnetic field enhances the compulsory convection resulted from the coupling between the surface tension gradient and the buoyant force. The convection heat transfer in the molten pool gradually enhances with the increase of magnetic field intensity, which makes the temperature gradient of the molten pool gradually decrease and the solidification rate gradually increase. As a result, the solidification structure of the alloy forming part is gradually refined, and the friction and wear performances are enhanced.