The invention discloses a water-based cutting fluid used in slice machining of a hard brittle crystal material, comprising the following compositions: base oil, organic amine, organic acid, synthetic ester, emulsifier, rust and corrosion inhibitor, dispersion flocculation and hard-water resisting agent, antiseptic bactericide, antifoaming agent, coupling agent, adjuvant and a proper amount of water. Aiming at the machining techniques of inner circle slicing and outer circle slicing, the water-based cutting fluid has good generality, lubricity, strong antirust property and a long service life; when the cutting fluid is used, the machining situation can be observed easily, the cooling property is good, the cleaning and the permeability are strong, cuttings subside easily, and the machining problems of machining error, crack and the like caused by cutter blocking, abrasion and the like are solved; according to the invention, the problems that cuttings are easy to adhere and suspend, a cutter is seriously abraded, a machined product cracks easily, the accuracy is not high, the machining efficiency is low, the later period cleaning is complex and the like existing in the current slice machining of the hard brittle crystal material are solved, and further, a machine bed is protected, and the cost is reduced.