The invention provides a preparation method of thymol crystals, which comprises the following steps: after mixing eucalyptus oil and silica gel, performing a cyclization reaction until by sampling inspection, a refraction coefficient of a sample at a temperature of 20 DEG C is 1.4720 to 1.4723 and the sample has fragrance of isopulegol, and discharging to obtain the isopulegol; and after gasifying the isopulegol, performing a dehydrogenation reaction at a temperature of 200 to 300 DEG C by a copper-nickel catalyst to obtain liquid thymol crystals of which a refraction coefficient at a temperature of 20 DEG C is 1.4850 to 1.5100. Compared with the prior art, the preparation method has the following beneficial effect that the thymol crystals, which are prepared by the method, overcome the harsh high-temperature and high-pressure conditions of a conventional friedel-crafts alkylation reaction, are mild in reaction temperature, use the recyclable catalyst and are environmental-friendly, are high in purity, and are kept stable in gloss, unchanged in odor and slightly reduced in content in the storage period.