The invention discloses a supported foam magnesium catalyst used for acrylonitrile removing by supercritical water oxidation. Foam magnesium is cleaned and then modified by AgNO3 to prepare a substance M; the substance M is modified by a mixed liquor to prepare a substance N, wherein the mixed liquor is prepared from polyvinylpyrrolidone, methyl alcohol, trihydrate ruthenium trichloride, concentrated hydrochloric acid and sodium borohydride; the substance N is modified by a mixed liquor to prepare a substance O, wherein the mixed liquor is prepared from ZnC12, CuC12, concentrated nitric acid, urea and ammonium fluoride; the substance O is modified by a mixed liquor to prepare a substance P, wherein the mixed liquor is prepared from tetra-n-butyl titanate, tetraethyl orthosilicate and acetylacetone; the substance P is modified by a mixed liquor by a mixed liquor to prepare a substance Q, wherein the mixed liquor is prepared from PtC14, CoC12 and InC13; and the substance Q is modified by a mixed liquor to prepare the supported foam magnesium catalyst used for acrylonitrile removing by supercritical water oxidation, wherein the mixed liquor is prepared from PbC12, A1C13, NaOH, Na2CO3 and MnC12.