The invention belongs to the technical field of lithium ion batteries, and especially relates to a composite lithium ion battery ternary cathode material and a preparation method thereof. The composite lithium ion battery ternary cathode material includes a Ni-Co-Mn ternary cathode material as a core and a cladding layer that coats the Ni-Co-Mn ternary cathode material. The characteristic general formula of the Ni-Co-Mn ternary cathode material is LiNi<1-x-y>MnCoO2, 0.98 <= m <= 1.10, 0.2 <= x <= 0.25 and 0.2 <= y <= 0.25; the cladding layer is composed of Al2O3 and LiAlO2. The composite lithium ion battery ternary cathode material has high volume energy density and has better cycle performance at high temperature and high rate. In the preparation, organic aluminum alcoholate serves as an aluminum source and is firstly uniformly mixed with a cathode material; and then water is added to initiate the reaction to produce the ternary cathode material. The preparation method is simple and low-cost and has good cladding effect.