A kind of manganese oxide which could purify seawater arisesed at the historic moment. Lithium manganese oxide was synthesized by coprecipitation method and the manganese oxide was prepared by extracting lithium from lithium manganese oxide. The characteristics of the manganese oxide were studied by X-ray diffraction and transmission electron microscopy. The characteristics showed that the manganese oxide was pure spinel structure and about 100 nm in length. There were experiments to study the effects of oil and phenol removal in the seawater. At the same time recycling lithium from seawater was tested. The results showed that the manganese oxide could not only remove oil and phenol but also recycle lithium.