The present invention discloses a high energy density lithium-ion battery positive electrode material and a preparation method thereof, the high energy density lithium-ion battery positive electrode material is a multi-layer composite structure, the chemical formula is xLi2MnO3.(1-x) LiMO2 / yLiNi0 .5Mn1.5O4, wherein x is greater than 0.2 and less than 0.7, y is greater than 0 and less than 1, and M is one or more than one metal selected from Ni, Co, Mn and Al. The positive electrode active material is obtained by synthesizing a precursor by coprecipitation, then mixing the recursor with a lithium source, and sintering. The high energy density lithium-ion battery positive electrode material has both high capacity of a Li2MnO3 based lithium-ion battery positive electrode material and high voltage platform and good electrical conductivity of a lithium nickel manganese oxide material, thus showing higher energy density, excellent electrochemical properties. The preparation method is simple, and easy to operate, and the high energy density lithium-ion battery positive electrode material can be mass-produced.