A soft brittle functional crystal grinding and machining method belongs to the technical field of soft brittle functional crystal machining, and particularly relates to a soft brittle functional crystal ultra-precision grinding and machining method for a semiconductor and a photoelectric crystal. The method is characterized in that a micro-powder diamond segmental variable speed feed and a soft abrasion grinding wheel chemical mechanical grinding method are adopted to machine the soft brittle function crystal. During the crude grinding period and the accurate grinding period, the feeding speed of the grinding wheel is firstly high and then low. A grinding fluid is de-ionized water. A soft abrasive agent grinding wheel is adopted to conduct the chemical mechanical grinding. The soft abrasive agent is a macromolecule polymer or waterproof resin. A filler is NaHCO3 or a refined naphthalene foaming agent. The chemical and mechanical grinding fluid is adopted as a reaction fluid and a cooling fluid. The grinding fluid mainly contains lactic acid, acetic acid and de-ionized water. The pH value of the grinding fluid is 2-4. The invention has the advantages of high grinding and machining efficiency, low machining cost and high surface precision. In addition, no surface/sub-surface damage is caused to the surface of a workpiece, such as small scratches, embedment of free abrasive agent, plastic deformation, residual stress, and the like.