<p>To enhance the activity of polysaccharides, the method of ultrasound-assisted pectinase was adopted in this experiment to obtain Lizhi seed polysaccharide (LSP). And the derivatization of LSP was carried out by using the acetic anhydride method, chloroacetic acid carboxymethylation method, and phosphorus oxychloride phosphorylation method. The antioxidant activity of the obtained acetylated polysaccharide (Ac-LSP), carboxymethylated polysaccharide (CM-LSP), and phosphorylated polysaccharide (P-LSP), respectively, was investigated. The results showed that CM-LSP had good efficacy in antilipid and scavenging hydroxyl radicals, and its activity was even better than that of LSP and LSP-2. To explore the elaborate structure of LSP, pure LSP-2 was obtained by using diethylaminoethyl (DEAE)-52 cellulose column chromatography and Sephadex G-100 gel column chromatography in this experiment. The Congo red experiment on LSP-2 revealed that it did not possess a triple helix structure. Subsequently, infrared and nuclear magnetic resonance (NMR) analyses were performed on LSP-2, and it was found that LSP-2 had five sugar residues. The sugar residues contained α-1,3-, α-1,4-, and α-1,6-glycosidic bonds. Among them, there was also a furan type with β-1,4 and β-1,6 reducing aldose residues. Therefore, in general, this experiment achieved a high degree of purification of LSP, explored the rich structure of LSP-2, found the optimal conditions for the derivatization of LSP, and enhanced the antioxidant activity of LSP.</p>