The invention discloses a proportioning and preparing method for producing graft starch and particularly relates to composite modified graft starch for textile slurry. The composite modified graft starch for the textile slurry comprises the following raw materials in parts by weight according to proportion: 900 parts of acetate starch, 80-100 parts of alcohol, 40 parts of acrylamide, 10 parts of acrylic acid, 40 parts of vinyl acetate, 5 parts of caustic soda flakes, and 0.2-0.5 part of composite catalyst; and the requirements on the viscosity of the acetate starch is that L (Low Viscosity) is 4.0-0.0mPa.S, M (Middle Viscosity) is 8.1-12.0mPa.S, H (High Viscosity) is 12.1-16.0mPa.S, and the degree of substitution is 0.10+/-10%. According to the invention, a small amount of alcohol is adopted for production for the first time in China, and two problems to be solved on no utilization of water resources and no wastewater emission and treatment are solved. Ternary mixed monomers are grafted onto starch macromolecules under the condition of the alcohol solvent. Therefore, the serous coat performance of starch slurry is greatly improved, the requirements of textile manufactures on the starching of high-count polyester/cotton varieties, and the composite modified graft starch can replace PVA (Polyvinyl Acetate) to perform environment-friendly starching.