The invention provides a magnetic Co nanowire/porous alumina composite film and a preparation method of the magnetic Co nanowire/porous alumina composite film. The composite film comprises a Co nanowire and a porous alumina film, wherein the Co nanowire is located in a nano hole of the porous alumina film, and the length of the Co nanowire is reduced from the center of the porous alumina film to the periphery. The magnetic Co nanowire/porous alumina composite film has high-saturability annular structure colors and gradually-varied magnetism; the preparation method is simple, all that is required is one-time alternating-current electro-deposition, and the magnetic Co nanowire/porous alumina composite film can be prepared; the magnetic Co nanowire/porous alumina composite film is lower in cost and can be applied to the fields of anti-counterfeit, drawing, decoration, cosmetics, a development technology, dye sensitization, a solar cell and the like.