AbstractDimer acid (DA) was grafted onto lignin (EHL) to form a graft copolymer DA-g-EHL. The selection of the reaction type and the optimization of the reaction conditions for the grafting reaction were conducted through orthogonal and single factor experiments. FT-IR and thermal analysis were used to characterize the graft product. It was found that, compared with free radical grafting, DA can be grafted onto EHL more effectively by ester condensation with strongly acidic cation exchange resin as a catalyst. Under optimum reaction conditions, the increase of acid value and the yield of graft copolymer can reach about 9.3% and 83%, respectively. The application of DA-g-EHL in preparing modified phenolic aldehyde amine curing agent (PAA) was studied. Results showed that the flexibility of the epoxy resin cured by DA-g-EHL modified PAA is significant higher than that of the resin cured by EHL modified PAA. The graft of DA onto EHL may reduce the rigidity of EHL and the chain stiffness of the PAA modified by EHL.