The acylated tannin was prepared with tannin as the raw material, the reaction conditions were optimized, and the acylated tannin was used as a tanning agent for leather tanning, the performance of the tanned leather was measured and the tanning mechanism was inferred. The results showed that by dissolving 1 g of tannin in 6 g of trifluoroacetic acid, adding 1.5 g of hexamethylenetetramine, and reflux reaction for 1.5 h at 100 ℃, the acylated tannin with an aldehyde group content of 2.33 mmol/g was prepared. The shrinkage temperature of leather tanned with acylated tannin can reach to (85.3±2.5) ℃, the tear strength is (47.7±1.3) N/mm, the tensile strength is (10.2±1.1) Mpa, and the elongation at break is 97.9%±2.0%, which is better than unmodified tannin tanned leather. In addition, the tanned leather also has strong anti-ultraviolet capability. Acylated tannin retain their original active groups, which can form hydrogen bond cross-linking with leather collagen fiber molecules during the tanning process. The introduced aldehyde groups can react with the amino groups in the collagen fiber molecules to form stable chemical bonds. Under the combined influence of the two forces, the performance of the leather is significantly improved.