A lithium-ion battery positive electrode material iron manganese magnesium magnesium phosphate and its preparation method, the chemical general formula of iron manganese magnesium magnesium lithium phosphate is LiFe0.5Mn0.45Mg0.05PO4, the molar ratio is 1.0-1.2:0.5:0.45:0.05:1 Get lithium source, iron source, manganese source, magnesium source, phosphorus source in the ball mill tank, add carbon source, use distilled water as dispersant, place ball mill in ball mill, make iron manganese magnesium magnesium lithium precursor; Put it into a microwave oven, dry it with microwave treatment for 2~20 minutes, and then leave it for calcination; the precursor powder after microwave treatment is controlled at a heating rate of 5~12°C / min under the protection of an inert gas atmosphere, at 500~850 Calcining at ℃ for 3~15h, and finally cooling to room temperature with the furnace, the doped carbon-coated iron, manganese, magnesium, and lithium cathode materials can be obtained.