The invention discloses a fur tanning agent based on isocyanate and a preparation method of the fur tanning agent. According to the process, diisocyanate molecules are adopted to perform end-capping on two ends of fat-diol or polyether glycols in a mixed solution under the condition of catalysis, and prepolymer with high selectivity and high conversion rate is obtained; then, hydrosulphite is adopted to protect isocyanate groups on two ends of the prepolymer, the water solubility of the compound is increased, and the tanning agent is obtained finally. The molecular weight of the tanning agent can be controlled, thereby being applicable to tanning of furs with different fibrous textures; during the tanning process, the fur tanning agent can effectively permeate to the position among the fur collagen; hydrosulphite decomposes under the condition of raised temperature and raised alkali, the tanning agent molecule expose the isocyanate group to cause the isocyanate group to react with the amino group on the collagen molecule, crosslinking of the fur collagen is realized, and the purpose of tanning is achieved; only neutral salt is released during the entire process, heavy metal ions or free aldehyde groups are not generated, the process is simplified, the fur tanning agent is environment friendly, the price is low, the fur tanning agent is very suitable for fur, especially tanning of rabbit fur with light color.