The work aims to reduce the serious CO2/H2S corrosion problems of water injection on offshore oilfield in the Bohai sea and the serious pittig corrosion of the test coupons. An imidazoline quaternary ammonium salt LH-31 applicable to the oilfiled conditions was synthesized and the optimum corrosion inhibitor was obtained through molecule compound. At 95 ℃ and 0.15 MPa CO2 (including 100 μg/g H2S), the blank average corrosion measured at total pressure of 0.5 MPa N2 and flow velocity of 1.5 m/s for 72 hours was 2.0101 mm/a and pitting corrosion rate was 1.1106 mm/a. The average corrosion decreased to 0.1000 mm/a and pitting corrosion rate reduced to 0.5789 mm/a after 40 mg/L LH-31 was added. LH-31 had an absorption peak in 879.31 cm-1. A stretching vibration might occur between the nitrogen atoms and adjacent carbons and the quaternary ammonium salts might be formed. LH-31 had synergistic effect with thiourea and sodium benzoate. The efficient compound corrosion inhibitor 2E composed of 4% thiourea and 3% sodium benzoate could reduce the average corrosion to 0.0200 mm/a and the pitting corrosion rate to 0.0751 mm/a. The polarization curve test showed that the corrosion inhibitor 2E was the anode type of corrosion inhibitor. After the corrosion inhibitor passed the bypass corrosion test for 1 day, the average depth of the test coupon was reduced to 0.0258 μm from 0.139 μm after adding 40 mg/L of 2E. The corrosion inhibitor can effectively inhibit the uniform corrosion and pitting corrosion of mixed water injection in the Bohai Sea.