The invention relates to an ultra-fine denier polyester fiber and a preparation method thereof, and an ultra-fine denier polyester fiber. The ultra-fine denier polyester fiber mainly consists of polyester melt spinning through spinneret holes, oiling agent and high-speed It is prepared by winding; the oil agent of the oiling agent includes at least fluorine-containing polyether, and the mass of the fluorine-containing polyether is greater than or equal to 45% of the mass of the oil agent; the fluorine-containing polyether is ethylene oxide, [(1,1,2,2-tetrafluoroethoxy)methyl]random copolymer of ethylene oxide and propylene oxide, its structural formula is: the preparation method of the present invention is to measure polyester, extrude Extraction, cooling, oiling, drawing, heat setting and high-speed winding to produce ultra-fine denier polyester fibers. The ultra-fine denier polyester fiber of the present invention has excellent characteristics such as soft hand, high flexibility, soft luster, high water and oil absorption, high density, high specific surface area, high thermal insulation performance, etc., and with the development of weaving, printing and dyeing technology, its More potential features are also being discovered day by day.