Eight kinds of graft copolymers phenolic acid-g-polymanturonic acid (PA-g-PM) were prepared by graft copolymerization of eight phenolic acids (PA) and intermediate PM-EDA, which was synthesized from the reaction between the carboxy group of polymanuronic acid (PM), a hydrolysate of sodium alginate, and the amino group of ethylenediamine (EDA). The preparation conditions of PM-EDA were optimized， and the influence of PA structure on the antioxidant activities of PA-g-PM were investigated. The results showed that under the conditions of n(PM) ∶ n(EDA) =1 ∶ 1.5, n(PM) ∶ n(carbodiimide) ∶ n(N-hydroxysuccinimide)= 1∶2∶1, pH 8.0, and reaction time of 24 h, PM-EDA with the highest substitution degree of 17.27% was obtained. Data from UV-Vis, FTIR and XRD analyses showed that phenolic acids were successfully graft onto PM, with grafting rates ranging from (4.171±0.16) to (8.880±0.32) mg gallic acid/g for the eight copolymers. Compared with PM, all eight PA-g-PM copolymers exhibited significantly enhanced 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging rate and iron reducing power. The antioxidant performance of PA-g-PM was positively correlated with the number of phenolic hydroxyl groups contained, with that of PA-g-PM possessing para-phenolic hydroxyl group superior to that of possessing ortho-phenolic hydroxyl group. Meanwhile, PA-g-PM with methoxy group in replacement of phenolic hydroxyl group showed weakened antioxidant activity.