Abstract: The synthesis and properties of cationic cassava starch graft copolymer was studied in this paper. The cassava starch, acrylamide and dimethyl diallyl ammonium chloride（DMDAAC）was used as raw materials, and ammonium persulfate, sodium hydrogen sulfite (（NH4）2S2O8-NaHSO3) as the initiator system, determined the optimum formula for cationic cassava starch flocculant in dealing with the suspension of kaolin and diatomite. Furthermore, kaolin and diatomite suspension were used as a water treatment system to test the flocculating ability. The results showed that the optimum the dosage of flocculant was 4mg/L. Focculation would work when the pH of treating water system was 4-10, but the optimum pH was 7. After infrared analysis and the positive ion measured, we found that the obtained polymers were the positive ion starch grafting acrylic amide copolymer.