The invention relates to an offshore oilfield water injection well online one-step-process acidification technology, which comprises following steps of (1) preparing one-step acid liquor, wherein the one-step acid liquor consists of following components by weight percent: 20 to 40 percent of industrial acetic acid, 15 to 40 percent of industrial hydrochloric acid, 3 to 15 percent of deposition inhibitor, 5 to 15 percent of discharging aiding agent, 3 to 15 percent of corrosion inhibitor, 5 to 15 percent of antiswelling agent, 5 to 15 percent of iron ion stabilizer and 2 to 10 percent of hydrogen fluoride; (2) storing the acid liquor into an acid storage tank of an injection system, wherein the injection system consists of the acid storage tank, an acid resistant pump, a motor, a frequency converter, a flow sensor, a pressure sensor, a computer, a check valve, a three-way pipe and an acid injection pipe; (3) utilizing the acid resistant pump to pump the acid liquor into the water injection flow; (4) monitoring the variation of the pressure and the flow of the injected acid, and calculating skin coefficients in real time; and (5) keeping the water injection after stopping the acid injection. Due to the adoption of the offshore oilfield water injection well online one-step-process acidification technology, high efficiency, rapidness and large scale in plug removal of an offshore oilfield water injection well can be realized, the water injection pressure can be reduced, and the water injection quantity can be improved.