The invention relates to a preparation method of a florfenicol nanocrystal. The preparation method comprises the following steps of under the stirring function, preparing a saturated solution of florfenicol at the temperature of 40 to 60 DEG C; adding methyl cellulose into the florfenicol, and completely dissolving the methyl cellulose solid after 0.5 to 2h at the constant temperature, wherein themass percentage of the methyl cellulose is equal to 0.5 to 2% of the florfenicol; cooling to the temperature of 10 to 20 DEG C at the cooling rate of 0.1 to 1 DEG C/min, culturing the crystal for 0.5to 2h at constant temperature, separating the obtained suspension liquid, and drying, so as to obtain the florfenicol nanocrystal. When the prepared florfenicol nanocrystal is observed under an SEM (scanning electron microscope), the florfenicol nanocrystal is a nanorod-shaped crystal with diameter of 100 to 900nm. The preparation method has the advantages that the used dispersant, namely the methyl cellulose, has good biodegradability, biocompatibility and bioabsorbility, any environment protection problem is not left after degrading, and the toxicity, sensitivity and irritation are avoided;the cost of the required raw materials and solvents is low, and the preparation process is simple and rapid.