The invention discloses a lithium-rich manganese-based high-energy-density lithium-ion battery and a preparation method thereof. According to the battery, with a lithium-rich manganese-based material doped with lithium vanadium phosphate as a positive electrode and a mixed material of a nano silicon-carbon material and graphite as a negative electrode, through optimal configuration of a conductive agent, a diaphragm, an electrolyte and the like, the influences to the battery performance due to the structure change of the lithium-rich manganese-based material and silicon volume expansion in a cyclic process are effectively reduced. The preparation process of the battery comprises the following steps: active material premixing, pulping, film production, winding/laminating, packaging, liquid injection, secondary packaging, formation, inspection and the like. The battery has the advantages of high energy density, high safety coefficient, relatively long cycle lifetime and the like and can be used as a dynamical chemical power source.