The invention discloses a binder-free polycrystalline cubic boron nitride abrasive and a preparation method thereof. The preparation method of the abrasive comprises the following steps: 1, adding hexagonal boron nitride powder into a die, and carrying out molding preformation; 2, carrying out cold isostatic pressing treatment on the above preformed product under vacuum conditions; 3, adding the above obtained hexagonal boron nitride block into a tantalum cup in order to form an assembled block together with a pressure transmitting medium; and 4, carrying out high temperature high pressure synthesis treatment on the assembled block, carrying out furnace cooling, demolding, crushing, and grading to prepare the abrasive. The binder-free polycrystalline cubic boron nitride abrasive has the ultrafine crystal structure of submicron dimension cBN monocrystal, and the micro-fragmentation self-sharpening effect and the isotropy of the binder-free polycrystalline cubic boron nitride abrasive overcome passivation, cleavage, anisotropy and other defects of monocrystalline cBN abrasives; and the binder-free polycrystalline cubic boron nitride abrasive disclosed in the invention has high hardness and strength, is suitable for high precision processing of high hardness quenched steel, especially electrical, electronic and optical devices, has very high social values and economic benefit, and is suitable for popularization and application.