The invention belongs to the technical field of chemical catalysis and discloses a method for preparing a magnetic heteropoly acid salt catalyst by cationic modification. According to the method, a Fe3O4@SiO2 magnetic nano material is obtained through redox reaction of substances such as ferric chloride hexahydrate, trisodium citrate and ammonium acetate; a heteropoly acid material is obtained through self-assembly of antimony sodium tungstate and zinc acetate in water solution and interaction with positive and negative ammonium ions; the magnetic heteropoly acid salt catalyst is formed by combination of action of cationic surfactant CTAB (cetyl trimethyl ammonium bromide) and electrostatic interaction of positive and negative ions. The magnetic heteropoly acid salt catalyst synthesized according to the method is great in magnetism and beneficial to later-stage composite catalyst separation, recovery and recycling.