Quaternary phosphonium cationic starch (St-g-AM/ATPPB) flocculant was synthesized by using corn starch and acrylamide (AM), allyl triphenyl phosphonium bromide binary (ATPPB) through simultaneous gamma-irradiation. The chemical structure of the copolymer was characterized by FTIR (Fourier transform infrared spectroscopy) and 1H NMR (nuclear magnetic resonance) technologies. The effects of the absorbed dose in the range of 1 to 6 kGy, AM/ATPPB ratio and starch/(AM + ATPPB) ratio on grafting and flocculation were investigated. At a starch:AM:ATPPB wt ratio of 1:1.4:0.93, the graft ratio of AM/ATPPB reached maximum values at 3 and 6 kGy, and the cationic degree of St-g-AM/ATPPB increased with absorbed dose, while the turbidity reduction reached a maximum value at 5 kGy. At 3 kGy using a starch:(AM + ATPPB) wt. ratio of 1:2.33, the graft ratio of AM and ATPPB increased with AM/ATPPB ratio, while the cationic degree and turbidity reduction reached maximum values at AM:ATPPB wt. ratios of 1:0.67 and 1:0.5, respectively. At a constant AM:ATPPB wt. ratio of 1:0.67 and 3 kGy, the grafting ratio increased with starch/(AM + ATPPB), while the cationic degree and turbidity reduction simultaneously reached maximum values at a starch:(AM + ATPPB) wt. ratio of 1:2.67. Further study revealed that the higher the absorbed dose or the lower the starch/(AM + ATPPB) ratio, the lower the optimal flocculant concentration.