Abstract: In this paper, oxidized polysaccharides (OP) with different relative molecular weight were coordinated with Al-Zr tanning agent (AZ) to prepare OP-AZ tanning agents, and the tanning performance of complex tanning agents were investigated. The effect of relative molecular weight of oxidized polysaccharide (OP) on tanning performance of OP-Al-Zr complex tanning agents (OP-AZ) was investigated. The OPs with similar carboxyl group content (about 5.7 mmol/g) but different relative molecular weight were prepared using polysaccharides with different relative molecular weight under same oxidation conditions. Experiments demonstrated that the OP2 with a moderate relative molecular weight (Mw 3135) endowed AZ with good alkali resistance (precipitation pH higher than 4.0) and ensured that OP2-AZ can uniformly penetrate into pickled pelt and be fixed stably in collagen fibers, so that the shrinkage temperature of tanned leather reached 83.4 ℃, resulting in better physical properties of crust leather. OP1 with a high relative molecular weight (Mw 37 363) hindered the penetration of complexes in pelt, while OP3 (Mw 568) and lactic acid (relative molecular weight 90) with low relative molecular weight were not conducive to the cross-linking of the complexes in collagen fibers. Therefore, the appropriate relative molecular weight of ligand is very important in improving tanning performance of AZ tanning agents.