In this paper, hemicellulose extracted from APMP pulping waste liquid is used as raw material, carboxymethylated product (CMHC) and p-hydroxybenzoic acid is esterified in DMAc/LiCl homogeneous medium. Pyridine is used as a catalyst to synthesize a series of the carboxymethyl hemicellulose p-hydroxybenzoate (P-CMHC). In order to obtain the esterification product with higher degree of substitution, the response surface was used to optimize the synthesis conditions of the carboxymethyl hemicellulose p-hydroxybenzoate reaction: reaction temperature 79.08 °C, reaction time 4.98 h, and the amount of p-hydroxybenzoic acid (mol/mol) is 2.14, and the degree of substitution of P-CMHC synthesized is 0.232 under this process condition; the functional group analysis of the synthesized product by FT-IR finally proves the successful synthesis of carboxymethyl hemicellulose p-hydroxybenzoate. And its synthesis mechanism was analyzed; the biological activity of P-CMHC was explored, and it was found that it has good antioxidant properties and no cytotoxicity. In the future, it can be used as a new material in the fields of biomedicine and cosmetics.