The invention relates to a method for preparing a TiO2 / PS / Fe3O4 magnetic nanoparticle photocatalyst, comprising the following steps of: (1) preparing oleic acid modified Fe3O4 nanoparticles; (2) preparing an aqueous-phase magnetic fluid; (3) preparing styrene miniemulsion; (4) preparing magnetic polystyrene beads PS / Fe3O; and (5) preparing the magnetic photocatalyst TiO2 / PS / Fe3O4, which specifically comprises the following steps of: mixing absolute ethyl alcohol with tetra-n-butyl titanate, and performing magnetic stirring to form a solution A; adding the magnetic polystyrene beads PS / Fe3O to de-ionized water and performing ultrasonic treatment to form a solution B; under magnetic stirring, adding the solution A to the solution B, thereby obtaining sol after 30-40 min, wherein TiO2 covers the PS / Fe3O at the moment; after condensing and refluxing the sol in a water bath, filtering the sol to obtain the TiO2 / PS / Fe3O4, washing the TiO2 / PS / Fe3O4 by using ethanol, filtering, washing by using distilled water and filtering, thus obtaining a solid; and drying the solid until the weight thereof is constant, thereby obtaining the magnetic photocatalyst TiO2 / PS / Fe3O4 with the polystyrene PS as an isolating layer, the Fe3O4 as a magnetic core and the TiO2 as a shell. The product obtained by using the method is low in energy consumption, high in catalytic activity and recyclable.