In order to enhance oil recovery of water flooding oilfield and extend utilization of green raw materials such as cotton staple in oil exploitation，the hydrogel plugging agent was synthesized from cotton staple through preprocessing by hydrochloric acid freezing and resolving in 6% NaOH +4% urea mixed solution，and reacting with epichlorohydrin. The performance of the hydrogel plugging agent was evaluated by thermogravimetry，static mechanical tester，rheometer and oil production chemical evaluation instrument. The optimum conditions for preprocessing cotton staple were cotton staple 3 g，dilute hydrochloric acid 36 mL with mass fraction 3%，temperature 95℃ and heating time 120 min. The synthetic conditions of hydrogel plugging agent were as follows：reaction temperature 60℃，gelling time 5 h，cotton staple 3 g，crosslinking agent of epichlorohydrin 4 mL，6% NaOH+ 4% urea mixed solution 40 mL. The prepolymer synthesized under these conditions had low viscosity，and could be pumped into oil well easily. The plugging rate of the hydrogel could achieve up to 99.64%，the compressive strength was of 131 kPa. The cotton staple hydrogel plugging agent possessed better temperature and salt resistance and low cost.