A composite sodium ion battery cathode material with radial heterojunction and a preparation method thereof are provided. The cathode material is prepared by sintering a precursor and a sodium source, and the prepared cathode material is a core-shell structure with a radial heterojunction, the core of the cathode material is an O3-type nickel-manganese-based layered oxide, there is a coating material on the surface of the core, and the coating material is a P2-type nickel-manganese-based layered oxide. By adopting the above composite sodium ion battery cathode material with radial heterojunction and preparation method thereof, while reducing the cost of the materials, the composite cathode materials with radially heterogeneous distributions are synthesized by constructing different thicknesses of Ni.sub.0.3Mn.sub.0.7(OH).sub.2 on the surface of Ni.sub.0.5Mn.sub.0.5(OH).sub.2 precursor, so as to improve the air stability and cycling stability of the materials, and improve the sodium ion transmission kinetic property.