The microstructure and properties of Cu-Ni-Si-Mg alloys were studied by applying a travelling magnetic field during solidification. The results show that the microstructure of the alloy gradually changes to an equiaxed crystal shape, while the grains are refined with increasing magnetic field current, and the size of the dendrites decreases continuously under the action of a magnetic field. When the magnetic field current is 90 A, the grain size is the finest. The ingot is almost entirely composed of fine equiaxed crystals, and the grain size is reduced by approximately 94% to (0.5±0.19) mm compared to that of the alloy without the travelling magnetic field. The microstructure was observed by transmission electron microscope and the diffraction pattern was confirmed. The precipitated phase at the grain boundaries is δ-Ni2Si. Under the condition of a 60 A magnetic field current, the tensile strength of the alloy is the largest, the yield strength increases by 41.8%, and the tensile strength increases by 12.9%, while the hardness of the alloy is not affected by the travelling magnetic field. This paper describes the development of a scalable production technology for improving the strength of age-hardened Cu alloys by refining grains with a travelling magnetic field.