The invention relates to the technical field of lithium batteries, and discloses a cobalt-free lithium-rich manganese-based positive electrode material, a composite positive electrode sheet and a lithium ion battery, wherein the chemical formula of the cobalt-free lithium-rich manganese-based positive electrode material is xLi2MnO3.(1-x)LiMeO2+a, x is greater than 0 and less than 1, a is greater than or equal to 0 and less than 1, and Me is selected from more than one of Mn, Ni, Al, Mg, Ti, B, F, Y and La. When the cobalt-free lithium-rich manganese-based positive electrode material is used for preparing a lithium ion battery, the preparation process is simple, the energy consumption is low, the cost is low, the industrialization is easy, and the prepared battery has the advantages of highfirst effect, good rate capability, slow voltage attenuation in the cycle process and good cycle stability, and has commercial application value.