The invention relates to a ternary positive electrode material of a high-capacity lithium ion battery and a preparation method thereof. The ternary positive electrode material contains lithium, manganese, cobalt, nickel and other elements. The preparation method comprises the steps of preparing mixed salt solution of manganese salt, cobalt salt and nickel salt and adding polymer organic matters into the mixed salt solution, adding a precipitate complexing agent and a lithium compound into the solution to prepare a precursor sizing agent, stirring the sizing agent uniformly, preparing precursor particles through a spraying drying method, and roasting in air to obtain the high-capacity positive electrode material, namely LixNiyCozMn(1-y-z)O2, wherein x is more than or equal to 1.2 and less than 1.5, y is more than 0 and less than or equal to 0.5, z is more than 0 and less than or equal to 0.5, and y plus Z is less than 1. The LixNiyCozMn(1-y-z)O2 ternary positive electrode material has high capacity, stable electrochemical performance and simple technique process, is easy to operate, is environmentally friendly, and is suitable for large-scale industrial production and application.