The invention relates to the technical field of hard and brittle material ultra-precision machining. In order to achieve ductility domain grinding of the largest cutting depth, high surface quality of the hard and brittle material, and efficient and high-precision production machining, according to the technical scheme, a method and device for determining the grinding brittleness-ductility transformation critical cutting depth of the hard and brittle material comprises the following steps of (1) according to a grinding wheel used in ultra-precision grinding, determining the number, shape and distance of grains of a homemade multi-grain tool, (2) according to the grain shape, distance and front-angle parameters selected in the step (1), machining three or more grains on a grain base body, (3) controlling the multi-grain tool and a workpiece to generate relative motion, beginning to conduct scratching on the surface of the workpiece, meanwhile, controlling the multi-grain tool to conduct continuous feeding, enabling the depth of the scratch to continuously change, and observing and measuring the workpiece after scratching by utilizing a surface contourgraph and a confocal microscopy. The method is mainly applied to ultra-precision machining.