A preparing method of triethanolamine oleate used for cutting fluids is disclosed. The method includes a step of material preparing, namely a step of weighing oleic acid and triethanolamine in a mass ratio of 2:1, a step of stirring, namely a step of adding mixed materials and toluene together into a reactor and stirring, a step of heating, namely a step of heating the reactor until no water is generated, and maintaining the temperature, and a step of finished product cooling, namely a step of feeding cooling water, cooling until the temperature is 60 DEG C or below, and allowing nitrogen feeding to be stopped to prepare a finished product. By the manner, the preparing method of the triethanolamine oleate is provided. The triethanolamine oleate prepared by the method is added as an oiliness improver into cutting fluids, and allows a physical adsorption film to be formed easily between a cutter and a workpiece to be cut, thus playing a role of lubrication.