The invention relates to nano rodlike zirconium boride powder and a preparation method thereof. According to the technical scheme, the preparation method comprises the step of uniformly mixing, by weight, 27-39% of zirconium oxide powder, 8-16% of boron carbide powder, 4-8% of non-formalized carbon powder, 12-26% of sodium chloride powder and 24-36% of potassium chloride powder, so that a mixture is obtained; putting a crucible containing the mixture into a saggar, then placing the saggar in a microwave oven, preserving the temperature for 20-40 minutes under the conditions that the vacuum degree is 10-50 Pa, argon atmosphere is provided, and the temperature ranges from 1000 DEG C to 1200 DEG C, conducting natural cooling, then conducting cleaning with deionized water, and preserving the temperature for 6-12 hours at the temperature of 65-80 DEG C in a vacuum drying box, so that the nano rodlike zirconium boride powder is obtained. The preparation method has the advantages of being low in cost, simple in process, low in reaction temperature, high in reaction velocity, suitable for industrialized production and environmentally friendly, and the obtained nano rodlike zirconium boride powder is high in purity, of a uniform rodlike structure, controllable in dimension, large in specific surface area and good in sintering property.