The invention discloses a preparation method of a molybdenum-sulfur complex lubricating oil additive with high molybdenum content. The preparation method comprises the following steps: I, preparing a mixed liquid A from a hexavalent molybdenum compound, a sulfur-containing compound and distilled water, and then adjusting a pH value; II, adding dialkyl amine into the mixed liquid A which is subjected to pH value adjustment, then stirring, and then adding a phase transfer catalyst to obtain a mixed liquid B; III, under the protection of a nitrogen atmosphere, dropwise adding carbon disulfide into the mixed liquid B to obtain a mixed liquid C; and IV, standing the mixed liquid C till the mixed liquid C is layered, removing a water phase, washing an organic phase by virtue of an alkaline solution and an organic solvent, then distilling, and drying to obtain the high-molybdenum-content molybdenum-sulfur complex lubricating oil additive of which the mass percentage content of molybdenum is not less than 10%. The preparation method disclosed by the invention is simple in preparation process, and is favorable for industrial production; and the prepared molybdenum-sulfur complex lubricating oil additive has relatively high molybdenum content and good oil solubility, and after the additive is added into lubricating oil, the anti-wear and friction reduction properties of the lubricating oil can be greatly improved.