Terminal phenolic hydroxyl triazine dendrimer (HTHP) was prepared by putting cyanuryl chloride and 1,3,5-benzenetriol into acetone and then adding K2CO3 into the mixture as deacid reagent, which is called “One Step Process”. HTHP can be used in the leather tanning process as an additive. The result shows that adding HTHP into leather tanning process can improve the combination tanning property of leather collagen. When we add 10% (based on the mass of pickled skin) of HTHP, the shrinkage temperature of crust is 63.11°C and the mechanism of tanning is same as the vegetable tannin which has abundant of plant polyphenol. When the usage of HTHP is 10% and the aluminum salts is 2.5% (consider the mass of Al2O3), the shrinkage temperature of crust can reach to 82.2°C. It can be concluded that it shows synergism when there are both HTHP and aluminum salts in the process.