The invention belongs to the technical field of veterinary drug, and discloses a preparation method of a chitosan nano particle. The main component of the chitosan nano particle is an anti-parasite drug, the carrier of the chitosan nano particle is chitosan, the weight ratio of the anti-parasite drug to chitosan is 1:10 to 1:30, and the optimum range of the weight ratio of the anti-parasite drug to chitosan is 1:4 to 1:8. The prepared chitosan nano particle is in a round shape. The shape of the chitosan nano particle is normal. The chitosan nano particles do not agglomerate together. The particle size of the chitosan nano particle is small, and the particle size distribution is narrow. The results of experiments taking rabbits as the models have proved that: the anti-parasite drug embedded by chitosan is absorbed faster, and the bio-utilization rate of the anti-parasite drug is higher, compared to those of anti-parasite drugs in conventional dosage forms.