The invention discloses a magnetic compound-cyanide decomposing enzyme composite nanoparticle, its preparation method and application, and a method for treating electroplating wastewater by using it. The raw materials for making the composite nanoparticle include: spinel ferromagnetic nanoparticle, magnetic oxide Nanoparticles, SiO 2 Nanoparticles, cyanide decomposing enzyme, first polymer compound, second polymer compound, silane, amine compound, nucleoside monophosphate disodium salt, first inorganic metal salt, second inorganic metal salt; The stone ferromagnet is modified by amino groups, by wrapping SiO 2 The magnetic oxide nanoparticles are modified by epoxy groups, and then the metal adsorption coupling is carried out through the first inorganic metal and the enzyme, which increases the way and amount of enzyme coupling, and stabilizes the enzyme activity and applicable pH range and temperature range. At the same time, the magnetic substance can be used to absorb heavy metal ions or dyes and achieve the effect of decomposing cyanide, reducing the amount of flocculant used and the amount of sludge after treatment.