The invention discloses a preparation method of a ferrous disulfide-graphene composite nano lubricant. The preparation method is characterized by comprising the following steps: (1), adding matrix graphene, an iron source ferrous chloride and a dispersing agent into water to prepare liquor, regulating pH value to be alkaline, adding a vulcanizing agent thiourea, and uniformly mixing the liquor for future use; and (2), transferring the liquor into a stainless steel reaction kettle, sealing, cooling to the room temperature after carrying out hydrothermal reaction for 16 hours at 140 DEG C-240 DEG C, cleaning products by using deionized water, repeatedly cleaning by using absolute ethyl alcohol, carrying out vacuum drying to obtain black powder, wherein the structure of the black powder is that an FeS2 nanosphere coats on a graphene sheet layer, and dimension is 450 nm-800 nm. According to the preparation method, materials are easily available, price is low, preparation process is simple, parameters are easy to control, production process is safe and environment-friendly, and therefore, the preparation method is especially suitable for large-scale industrial production.