The invention provides a preparation method of manganese oxalate feed liquor and spinel lithium manganate. The method comprises the following steps: with manganese powder as raw material, producing manganese oxalate turbid liquor through two-step method of activating-corrosion precipitation, wherein inorganic acid and inorganic ammonium salt serve as dual catalysts in the activating step, and are used cooperatively; the method further comprises the subsequent oxalate corrosion precipitation process. Spinel lithium manganate is obtained by pouring the manganese oxalate turbid liquor into a ball-milling tank, adding lithium salt, performing ball-milling and then sintering. The process of synthesizing battery-grade manganese oxalate is simple and easy to implement, high in efficiency, low in cost, good in product crystal structure, less in impurity, uniform in granularity and suitable for industrial production. Manganese oxalate is used for preparing lithium manganite by directly using wet-process ball-milling and sintering, the method steps are simple, and prepared lithium manganite is spinel lithium manganite which is low in specific surface area, high in specific capacity, low in self-discharge rate, great in tap density, stable in product performance, and excellent in processing performance.