The invention discloses a composite flocculating agent and a preparation method thereof. With titanium white waste acid and ferrous sulfate, waste nylon and starch as raw materials, an efficient composite flocculating agent with less charging quantity, high coagulation speed, strong decontamination capability and wide application range is obtained by using a simple one-pot process. According to the invention, wastes are controlled by wastes, wastes are turned into wealth, the environment is improved, a new way is built for recycling and hazard-free treatment of industrial wastes and the composite flocculating agent and the preparation method thereof are beneficial for environmental protection. Through the adoption of the composite flocculating agent and the preparation method thereof, the titanium white waste acid and side product ferrous sulfate can be sufficiently utilized, an environment pollution problem is solved, the added value of the titanium white powder industry can be also effectively increased, the production cost of the flocculating agent and sewage treatment cost are reduced, and very strong economic benefits, environment friendly benefits and social benefits are obtained.