The ordinary flocculants rely on gravity settling for solid-liquid separation, it is difficult to meet the requirements of wastewater treatment in short flocculation settling time, few sludge production and low moisture content. A new type of flocculant is required to solve these difficulties. Magnetic materials are given more attention because of the magnetism in magnetic field. It is significant to prepare magnetic composite flocculant to realize solid-liquid separation with the combined effects of both gravity and external magnetic field. The gelatin-based magnetic flocculant consisted of poly (acrylamide-co-acryloyloxyethyl thimethylammonium chloride) grafted gelatin-coated Fe3O4 [magnetic poly (AM-co-DAC)-g-gelatin] was prepared in this article. A Box-Behnken design for determination of optimal plan with three-level and three-variable was designed. The best conditions of preparing magnetic poly (AM-co-DAC)-g-gelatin were determined. The structure of magnetic poly (AM-co-DAC)-g-gelatin was studied by infrared spectrum and its magnetic properties were characterized by magnetic hysteresis loop. The flocculation of magnetic poly (AM-co-DAC)-g-gelatin was compared with conventional flocculant. The flocculation experiment results showed that the magnetic poly (AM-co-DAC)-g-gelatin was effective for removal of turbidity and significantly reduced the settling time of flocs, compared to conventional flocculants.