The invention relates to a preparation method of a ZnFe2O4/ Ag3PO4 composite photocatalyst, belonging to the technical field of environmental material preparation. An ion exchange technique is adopted for the invention, and the preparation method comprises the steps of firstly, feeding FeCl3.6H2O solid and ZnCl2 solid into ethylene glycol, evenly stirring, then feeding sodium acetate (NaAC) and polyethylene glycol (PEG) into the mixture, continuously mixing, and carrying out a reaction to obtain ZnFe2O4 nanospheres; after that, feeding the ZnFe2O4 nanospheres into deionized water, rapidly stirring, dropwise adding a disodium hydrogen phosphate solution, carrying out a reaction, then dropwise adding a silver nitrate solution, stirring until reactants have a full reaction, washing the product, and then carrying out vacuum drying on the washed product to obtain the ZnFe2O4/Ag3PO4 composite photocatalyst. A composite visible light photocatalyst system is established; a heterogeneous structure system is established, so that the activity and the stability of silver phosphate can be improved; furthermore, the photocatalyst can be efficiently recovered by using the ZnFe2O4; the composite photocatalyst has a better degrading effect for the antibiotics in waste water.