The invention discloses a preparation method for gold magnetic composite nanometer particles. Firstly, magnetic nanometer particles of Fe3O4 are prepared by using a chemical co-precipitation method, the surface of the magnetic particles are modified by a silane coupling agent, then with the modified nanometer particles of Fe3O4 as seeds and oil amine as a reduction agent, gold ions are reduced to metallic gold on the surface of the magnetic nanometer particles in an ultrasonic condition, then the nanometer particles are prepared. The nanometer particles have the advantages of good dispersion, strong magnetic response and having double bonds that can lead to polymerization reaction on the surface of the particles. The nanometer particles have the diameter of 30 to 50nm and saturation magnetization of 6.8 to 17.6 emu per gram, thus showing super paramagnetism. The nanometer particles not only have magnetic response function, but also can carry out polymerization reaction with a plurality of functional monomers, thus having wide application prospect in a plurality of fields, targeted drug control and release, hyperthermia, isolation of protein and enzyme, etc.