Abstract: A new method for the preparation of trimanganese tetroxide (Mn₃O₄) by air direct oxidizing free divalent manganese was made at a certain temperature and pH value varying correspondingly according to the change of manganese ion concentration. The results show that the rate of oxidization of manganese hydroxysulfate is slower than that of manganese hydroxide, and the package of manganese hydroxysulfate is a basic reason for high sulfate content in Mn₃O₄. In comparison with the method in which Mn₃O₄ is prepared by air direct oxidizing the hydrolyzates of manganese, the proposed method can efficiently reduce impurity contents of Ca²⁺, Mg²⁺ and SO₄^2- in Mn₃O₄.