Polysaccharides extracted from Athyrium multidentatum (Doll.) Ching rhizome (AMCP) were chemically modified by introducing different functional groups. The acetylated, sulphated and phosphorylated derivatives were synthesized successfully, and characterized by IR spectra. The antioxidant activities of polysaccharide derivatives were explored employing various established in vitro systems. Acetylated derivative was prepared using N-bromosuccinimide (NBS) as catalyst. Polyphosphoric acid method was applied in phosphorylation reaction. The phosphorylated derivative exhibited the greatest antioxidant activity among all derivatives, and possessed much stronger reducing power than AMCP. Available data suggested that the substituted groups of AMCP played important roles on the antioxidant activity, and the mechanism on the antioxidant activity of acetyl, sulphonyl and phosphoryl group was indicated