The invention discloses a method for preparing lithium manganate by a wet-doping method. By the wet-doping method, manganese chloride solution, ammonium bicarbonate solution and lanthanum nitrate solution are reacted in aqueous solution. Lanthanum hydroxide and manganese carbonate are generated in a coprecipitation way. Manganic manganous oxide-doped crystals are obtained after high-temperature calcination, crushing and grading. Lithium carbonate and the manganic manganous oxide-doped crystals are mixed according to the molar ratio of lithium to manganese, and the molar ratio is 1.15:2. After being mixed effectively and uniformly, the above mixture is put into a kiln to be sintered at a high temperature. After cooling, crushing, grading, sieving and deironing of the sintered materials, spinel-shaped and lanthanum-doped lithium manganate is obtained. The method can combine materials and doping elements tightly. During the high-temperature calcination reaction, the doping elements can be inserted into crystal structures to be doped completely and fused with the doped crystals together. The doping of lanthanum element can raise the average oxidation state of manganese ions, inhibit Jahn-Teller effect effectively, reduce the capacity fading and raise the cycle performance.