The invention discloses a composite flame retardant and a preparation method thereof. The composite flame retardant is characterized by being formed by mixing and reacting a chitin compound, a phytic acid compound, acetic anhydride, siloxane, polyamine, white carbon black, urea and water, wherein the chitin compound is at least one of chitin or chitosan; the phytic acid compound is at least one of phytic acid, sodium phytate, potassium phytate, magnesium phytate, calcium phytate, iron phytate, cobalt phytate or zinc phytate. Preparation method of the flame retardant mainly relates to a three-stage mixed reaction process: firstly, uniformly mixing the chitin compound, the phytic acid compound, water, polyamine and white carbon black; then adding acetic anhydride and siloxane to be uniformly mixed; and finally, putting urea to be uniformly mixed, and discharging, cooling and crushing the mixture to obtain a product. The composite flame retardant disclosed by the invention not only can effectively retard flames of polyolefin and woods, but also has good compatibility with polyolefin and an amino resin aqueous coating, and can effectively avoid phenomena of blooming, white spraying, dialyzing and fracturing.