A novel method immunosensor based on chemically functionalized Fe(3)O(4)/Ag/Au nanocomposite was developed. Fe(3)O(4)/Ag seeds were coated with Au and Fe(3)O(4)/Ag/Au nanocomposites were obtained. The nanocomposites could be obtained without violent stirring and the operation was easy and convenient. The magnetic nanocomposites can be easily immobilized on Au film of the surface plasmon resonance (SPR) biosensor with a magnetic pillar. The immobilization of Fe(3)O(4)/Ag/Au nanocomposites on the Au film leads to a large shift in resonance wavelength, which is due to the increase of the sensing membrane thickness, high dielectric constant of Ag and Au particles, and electromagnetic coupling between Fe(3)O(4)/Ag/Au nanocomposites and the Au film. The effects of Fe(3)O(4)/Ag/Au nanocomposites on the sensitivity of SPR biosensor were also investigated. As a result, the biosensor based on Au film shows a response for dog IgG in the concentration range of 1.25-20.00 μg ml(-1), whereas the SPR biosensor based on Fe(3)O(4)/Ag/Au nanocomposites shows a response for dog IgG in the concentration range of 0.15-40.00 μg ml(-1). The sensitivity of the SPR biosensor based on Fe(3)O(4)/Ag/Au nanocomposites is higher than that based on Au film.