The invention belongs to the technical field of petrochemical engineering, and specifically relates to mini-emulsion used for thickened oil recovery and a preparation method thereof. The mini-emulsioncomprises the following ingredients in parts by weight: 40-60 parts of an organic solvent, 0-8 parts of a nonionic surfactant, 4-20 parts of an anion oligomeric surfactant and 120-150 parts of deionized water, wherein the anion oligomeric surfactant is an oligomer of a paralkyl phenol derivative. The mini-emulsion is prepared by the following steps: dispersing the nonionic surfactant and the anion oligomeric surfactant into the organic solvent and slowly dropping the deionized water into the mixed solution, so as to form the mini-emulsion. The obtained mini-emulsion does not have flash pointsand is good in viscosity reduction capacity.