The quaternary amino groups containing cationic starch with the degree of substitution 0.3903 was prepared via the reaction of potato starch with 3-chloro-2-hydroxypropyl-trimethyl ammonium chloride using predrying dry process. Then the cationic starch (CS) and poly ferric sulfate (PFS) would be used to treat potato starch waste water. And the cleaning efficiency of the chemical oxygen demand (COD) in the wastewater was studied by changing the experiment conditions. Though the orthogonal test, the optimal conditions were as follows: CS addition 0.15%, PFS addition 0.15%, initial reaction pH 7.0. The removal rate of COD was 64.31%.