Manganese dioxide (MnO2) is promising and widely used for catalytic oxidation, energy conversion/storage, environmental remediation, and biomedicine. Its synthesis has been extensively concerned, but strong oxidants, high temperature, and long reaction time are often required. Here we proposed the dissolved oxygen strategy for oxidization of Mn2+ at pH 12 to obtain MnO2 with green, high feasibility, and low cost. The oxygen dissolved in water was used as oxidant to oxidize Mn2+ ions at pH 12 for the formation of MnO2 heated at 55 °C for 1 h. The process is confirmed with the theoretical calculation from Nernst equation. The precipitate was collected and dried as MnO2 with small particles with large amounts of oxygen defective sites as validated with XPS, SEM, BET, and XRD results. Our MnO2 exhibits higher oxidation capacity than commercially available MnO2 as confirmed with the oxidization of methylene blue and streptomycin. By adjusting the temperature and pH, the organics were removed and MnO2 become Mn2+ ions again. Thus, we proposed a circulating water treatment strategy as the produced Mn2+ was transferred into MnO2 again after adjusted pH to 12 and thermal treatment. We hope the simple and green preparation strategy of MnO2 with low cost can expand the applications of MnO2.