The invention relates to a method for preparing a granular sepiolite modified waterproof gypsum board, and belongs to the field of preparation of gypsum board. The method is as below: sepiolite is crushed; the water swelling property of sepiolite is used that the sepiolite blocks the holes on a gypsum board after water absorption of the gypsum board; at the same time, polyethylene glycol and polyvinyl alcohol are employed in the preparation of the gypsum board, so that a polyvinyl alcohol film is formed on the surface to effectively cut off the water vapor. The invention has the beneficial effects that the gypsum board prepared by the present invention has the characteristics of light weight, waterproofness, thermal insulation and high strength; and the preparation method has simple steps and low cost.