The crude polysaccharide was extracted from hawthorn, and hawthorn polysaccharide (HP2) was obtained after separation and purification. The hawthorn polysaccharide selenium nanoparticle (HP2-SeNPs) was prepared by HNO₃-Na₂SeO₄ method, and its structure and antioxidant activity was studied. The selenium content of HP2-SeNPs was 4.91%. The results of the ultraviolet (UV), fourier-transform infrared (FT-IR), scanning electron microscopy (SEM), X-ray diffraction (XRD) showed that HP2-SeNPs was successfully synthesised. Particle size and Zeta potential analysis showed that the average particle size of HP2-SeNPs system was 137.85 nm, smaller than HP2. The Zeta potential absolute value of HP2-SeNPs system was greater than HP2, indicating that the stability of HP2-SeNPs system was higher than HP2. The scavenging capacity of HP2-SeNPs to DPPH, ·OH, and ABTS free radicals was concentration-dependent, and the highest scavenging rates were 69.77%, 60.81%, and 65.53%, which were significantly higher than HP2. It indicated that selenium modification could outstanding improve the antioxidant activity of hawthorn polysaccharide.