On-line diverting acids for continuous injection into water injection wells and methods of preparing the diverting acid are disclosed. The on-line diverting acids have a raw material composition of: 5.0-25.0% of hydrochloric acid, 3.0%-6.0% of ammonium fluoride, 4.0%-8.0% of a diverting agent, 3.0%-7.0% of a chelating agent, 1.0-3.0% of a corrosion inhibitor, 1.0-3.0% of an anti-swelling/swelled-clay-shrinking agent and a balance of water. The on-line diverting acids can remove plugging from deep zones of high and low permeability layers, and can be continuously injected to simplify the construction process, to achieve the purpose of reducing the pressure and stimulating the injection.