The invention relates to a lithium sulfur battery cathode material and a preparation method thereof, belonging to the functional composite material field. The invention provides a preparation method of a lithium sulfur battery cathode material, in particular, the elemental sulfur reacts with the mixed solution to form a reducing precursor liquid, and the reducing precursor liquid and the grapheneoxide solution are stirred so that the reducing precursor liquid is oxidized by oxygen-containing functional groups in the graphene oxide solution to generate elemental sulfur on the surface of the graphene in situ, and the obtained elemental sulfur adheres the graphene sheet layer tightly so that the elemental sulfur is encapsulated by the graphene; the graphene oxide in the graphene oxide solution is partially reduced graphene oxide; the mixed solution is hydrazine hydrate and N, N-dimethylformamide. The carbon-sulfur composite cathode material prepared by the invention has good interactionbetween graphene and elemental sulfur, and the elemental sulfur is well encapsulated between graphene sheets; the electrode material prepared by the composite has high specific capacity, good cycle performance and rate performance.