The invention discloses a method for improving magnetic property of high-entropy alloy by heat treatment through a constant magnetic field. By heat treatment through the magnetic field, the content ofa BCC phase in a FeCoNi(CuAl)0.8 high-entropy alloy microstructure is increased, and the magnetic property is improved. The purpose of improving the magnetic property of the alloy can be achieved without changing ingredients of the alloy. The high-entropy alloy is subjected to heat treatment through the magnetic field, the tissue structure is changed obviously, and the magnetic property is improved remarkably; and the heat treatment temperature is low relatively, and the magnetic property of the alloy can be greatly improved under the condition that magnetic field strength is small.