Abstract: The process of preparing high-purity new AB-PBO monomer：4-(5-amino-6- hydroxybenzoxazole-2-yl) benzoic acid（ABA）was studied，and the effect of pH on ABA molecular morphology and IR absorption were investigated. The results showed that ABA was conveniently synthesized from methyl 4-(5-amino-6-hydroxybenzoxazole-2-yl)benzoate (MAB) with purity of 99.5% and yield of 85% via alkali hydrolysis of K2CO3 at 60—100℃ and crystallization at pH 6.0 in aqueous solution of weak acid salt with reducing properties，such as Na2S2O4 or NaHSO3，which had the advantages of simple procedure and easy industrialization. The prepared ABA had the characteristics of high-quality，polymer retarder DMF-free，excellent thermo- and antioxydation -stability，and safe operation in polymerization. New progress was achieved in preparing polymer grade AB-PBO monomer.