The invention relates to a special oxidation furnace, and especially relates to a device for preparing magnesium oxide by magnesium manganese alloy. According to the invention, a currently-used crucible oxidation furnace containing blowpipes with a plurality of small holes is improved; the blowpipes with a small pore size are removed; an air box is divided into two or more which are directly communicated with a liquid surface; the ventilation area is large; the air-liquid contact area is increased by several times and even more; and the yield is greatly increased; the upper part in the furnace is partitioned into a pre-melting zone, a reaction zone, and a crystallization zone by the air box and alloy melt. Compared with original production devices, the invention has simpler process and smaller resistance; and by using the oxidation furnace with the same dimensions, the yield is increased by a factor, and the production cost is decreased by more than 45%.