Based on the CALPHAD (Calculated of Phase Diagrams) approach, series micro-scale amorphous/crystalline composite powders of (Fe0.75Si0.1B0.15)100-xCux (x = 30, 45, 55, 65, at.%) with core/shell microstructure were designed and successfully fabricated by using the gas atomization method. The obtained gas-atomized powder exhibits a Fe-Si-B-rich amorphous-core/Cu-rich crystalline-shell composite microstructure. Results indicate that the coercive force of the composite powders is almost the same as Fe75Si10B15 amorphous powder, but their saturation magnetization decreases with increasing Cu concentration. The formation mechanism of such amorphous-core/crystalline-shell composite powders is originated from the occurrence of liquid phase separation at high temperature and the different glass-forming ability (GFA) of the two separated liquids (Cu-rich and FeSiB-rich) during the rapid solidification process.