The invention relates to a cutting fluid used for metal processing and a preparation method thereof and in particular relates to a high-cleanliness economical microemulsion metal cutting fluid. The high-cleanliness economical microemulsion metal cutting fluid comprises the following raw material components in percentage by weight: 15-20% of base oil, 10-15% of alkaline retention agent, 5-10% of surfactant, 5-10% of antirust agent, 5-15% of emulsifying agent, 0.5-2% of corrosion inhibition agent, 5-10% of extreme pressure agent, 0.1-0.2% of defoaming agent, 1-2% of sterilizing agent and the balance of water. The preparation method of the high-cleanliness economical microemulsion metal cutting fluid comprises the following steps: adding the alkaline retention agent, the surfactant, the antirust agent, the emulsifying agent, the corrosion inhibition agent, the extreme pressure agent, the defoaming agent, the sterilizing agent, water and the like into base oil, and uniformly mixing and stirring. Compared with the prior art, the preparation method of the high-cleanliness economical microemulsion metal cutting fluid has the advantages that product quality and operating environment can be improved, processing efficiency is improved, durability of a cutter is enhanced, and cost for replenishing and managing cutting fluid is reduced, so that obvious economic benefit is obtained.