The invention discloses a composite high polymer flocculant for coking wastewater and a preparation method thereof. The composite high polymer flocculant adopts non-ionic modified starch, cationic modified starch, anionic modified starch, and amphoteric modified starch as the raw materials, in the presence of an initiator and under a weak acid condition, the raw materials undergo graft copolymerization with acrylamide, acrylic acid, methacrylic acid, acrylonitrile, acrylic ester, styrene, methylene acrylamide, maleic anhydride and other monomers to obtain a modified starch graft copolymer, and the copolymer is blended with an inorganic flocculant ferric sulfate, ferric silicate, ferric chloride, ferrous sulfate and ferrous chloride to obtain the composite high polymer flocculant. The composite high polymer flocculant has the characteristics of long molecular chain, compact floccules, wide source of raw materials, low price, biodegradability, and little secondary pollution, etc., and is a real efficient and environment-friendly flocculant. During use, the dosage of the composite high polymer flocculant accounts for 0.1-1% of the volume fraction of the treated coking wastewater, the COD removal rate of the treated coking wastewater reaches 85% or more, the decolorization rate is 95% or more, and the removal rate of suspended solids reaches 95% or more.