The invention relates to a method for producing mixed xylene from a crude benzene hydrogenation byproduct non-aromatic hydrocarbon and reformed C9<+> heavy aromatic hydrocarbon. The method comprises the following steps: preheating crude benzene hydrogenation byproduct, namely non-aromatic hydrocarbon, and introducing the byproduct into a dehydrogenation reactor, so as to obtain a first-stage hydrocarbon mixed product; processing the first-stage hydrocarbon mixed product in an aromatization reactor, so as to obtain a second-stage hydrocarbon mixed product; mixing the second-stage hydrocarbon mixed product with preheated reformed C9<+> heavy aromatic hydrocarbon with the same mass of the second-stage hydrocarbon mixed product, so as to obtain a third-stage mixed hydrocarbon product mainly containing mixed xylene; and separating the third-stage mixed hydrocarbon product by virtue of a gas-liquid separation tank, a debutanizer, a non-aromatic removal tower, a toluene removal tower and a xylene removal tower, so as to obtain mixed xylene. The method has the advantages that the dehydrogenation and aromatization of the crude benzene hydrogenation byproduct non-aromatic hydrocarbon are combined with a lightening technique of C9<+> heavy aromatic hydrocarbon, and hydrogen and benzene/toluene which are the byproducts of the dehydrogenation and aromatization of non-aromatic hydrocarbon are supplied to a heavy aromatics lightening section, so that an enterprise does not need hydrogen and benzene/toluene raw materials; and the raw material conversion rate and the yield of mixed xylene are very ideal.