Ascorbic acid and hydrogen peroxide were used as free radical initiators to induce the grafting of gallic acid onto sodium alginate, in order to enhance new properties of sodium alginate such as antioxidation and bacteriostasis, so that it could have potential in food preservation application. Folin-Ciocalteu method was used for determining the grafting degree of sodium alginate-gallic acid derivative. The results showed that the highest grafting degree was 4.260 mg/g when 5.54 mmol of hydrogen peroxide and ascorbic acid (molar ratio 3.51.0) was added. Infrared and Raman analysis showed that new peaks were appeared in the spectra of the derivative, indicating that gallic acid was successfully grafted onto sodium alginate. Differential scanning calorimetry showed that the thermal stability of the grafted sodium alginate was changed, and scanning electron microscopy showed that the surface of the derivative became rough. When sodium alginate-gallic acid derivative concentration was 4 mg/mL, the scavenging rate of scavenging 1,1-diphenyl-2-trinitrophenylhydrazine radical was 92.0%, which was close to that of gallic acid, far higher than that of sodium alginate. The results showed that the antioxidant activity of the sodium alginate-gallic acid derivative increased significantly. Better preservation of cherries was obtained by the coating of sodium alginate-gallic acid derivative than by the coating of sodium alginate.