Abstract: The optimum conditions of grafting copolymerization reaction of acrylic acid(AA) onto polypropylene(PP) fiber was studied by using benzoyl peroxide(BPO) as initiator.The primary and secondary sequences of the grafting copolymerization reaction conditions was analysed. The results showed that grafting copolymerization reaction was greatly influenced by the temperature，reaction time, concentration of BPO and that of AA in the primary and secondary role as the above sequence. The optimum reaction conditions of gaining higher exchange capacity for the weak acidic cation-exchange fiber are as follows: the grafting temperature 80 ℃, the reaction time 7 h, the concentration of BPO 2% and AA 80%.