The invention discloses a method for preparing nano diamonds by using a direct-current arc process, belonging to the technical field of preparation of carbon-related nano materials. The method is characterized in that direct-current arc hydrogen plasma is used as a heat source, graphite is used as a carbon source, nickel is used as a catalyst and silicon is used as a nucleation substance to synthesize diamond nano-particles. High-temperature hydrogen plasma is used for evaporating a block composite target material to form atomic and ionic states of raw material components, a silicon carbide cluster crystal nucleus is formed and atoms are induced to form a diamond phase during condensation, carbon atoms separated out from a supersaturated nickel-carbon solid solution become growth substances for the diamond phase, and a nano diamond blank is obtained through passivation. The remaining impurities including metals, graphite, amorphous carbon, silicon carbide and the like are removed through purification processes including acid treatment, high-temperature oxidation, rinsing and the like so as to obtain high-purity diamond nano-particles. The method disclosed by the invention has the effects and benefits that the preparation process is simple, the synthesis is performed under a normal pressure condition, and the low cost, low energy consumption and large-scale production of the diamond nano-particles are realized.