The invention provides a preparation method for a polyimide three-layer composite film with an electromagnetic shielding function. When carbon nanotubes are employed for modification of a polyimide film, the resistivity of the polyimide film can be effectively reduced; however, with increase of the content of the carbon nanotubes, the carbon nanotubes are substantially agglomerate in polyimide, which leads to drastic decrease of the mechanical properties of the polyimide film. The preparation method provided by the invention comprises the following steps: (1) preparation of a carbon nanotube-modified polyamide acid solution; (2) preparation of a carbon nanotube-modified polyamide acid microporous film by using an electrostatic spinning method; (3) preparation of a polyamide acid solution; (4) coating of two sides of the microporous film with the polyamide acid solution; and (5) thermal imidization of a prepared carbon nanotube-modified polyimide acid three-layer composite film. The polyimide three-layer composite film prepared in the invention is mainly applied to broaden the application scope of polyimide films in the field of electromagnetic shielding materials.