Taking soluble starch as the base material, the research introducing AM, the functional cationic monomer DMDAAC, hydrophobic monomer C16DMN+ and nano-SiO2 to synthesize a hydrophobically associating cationic starch-nano SiO2 composite polymer flocculant, and then the chemical structure of CSSADD was demonstrated by IR, XRD, TG and SEM. The reaction conditions were optimized by the method of single factor variable, and the study showed that the graft monomers molar ratio was AM:DMDAAC:C16DMN+=84.5:15:0.5, acidity control in PH value was 5~6, when nano-SiO2 and starch quality ratio scale was 1:3, starch andmonomers quality ratio scale was 1:3, the mass of initiator was 0.5% of total monomers, the reaction temperature was 45℃, and the reacting time was 3 hours, the CSSADD’s performance was best. The performance test indicated that when the PH value of kaolin suspension was 6 and the mass of CSSADD was 3mg/L, the upper transmittance approached 98.3%.