The invention discloses a preparation method of graphene modified finished product lubricating oil. The method comprises the following steps: preparing a graphene concentrated solution in parts by weight: preparing 89.5 to 99.9 parts of basic oil, 0.1 to 5 parts of graphene powder, 1 to 5 parts of polyamide wax powder, and 0.5 to 3 parts of oleic acid or mixture of one or more of the oleic acid, span 60, span 80, tween 20, silane coupling agent KH550 and polyethylene wax into mixed slurry, and pre-mixing the mixed slurry by virtue of an emulsification machine; emulsifying and dispersing the pre-mixed slurry in a controlled-temperature manner in a multilevel emulsification device; homogenizing and circulating the emulsified and dispersed slurry by virtue of a high-pressure homogenizing device; obtaining a graphene concentrated precursor solution; and adding the concentrated precursor solution into finished product oil, and uniformly mixing, thus obtaining the uniform graphene modified finished product lubricating oil. According to the preparation method, the components and a weight part ratio of the concentrated precursor solution are optimized, so that the concentrated precursor solution is uniform and stable in dispersion; and moreover, when the prepared lubricating oil is mixed in a great amount of finished product oil, excellent dispersion stability can still be maintained, and the modification effect for the finished product oil is significant.