Hydrogel is a new functional polymer material with a three-dimensional network structure. Hydrogels are widely used in drug release control, adsorption materials, biomimetic engineering, fiber optic sensing, tissue engineering, smart wear and other aspects, due to its injectability, good histocompatibility, non-toxic side effects, biodegradability in vivo, and environmental sensitivity. The sensing of the hydrogel is mainly caused by the change of external environment factors, causing the difference of swelling degree, which causes the water molecules to fill the inner pores of the gel to different extents, showing the process of the change of the overall refractive index of the hydrogel. In order to study the effect of different swelling degree on refraction index of the hydrogel, in this paper, chitosan was used as raw material, ammonium persulfate, N and N-methylenediacrylamide were used as an initiator and crosslinking agent respectively, and grafted with acrylic acid, and chitosan hydrogel was prepared chemically in a nitrogen environment. By designing the measurement structure and building the experimental platform, the swelling properties of chitosan hydrogel were tested, and the swelling degree of the gel was calibrated; the transmission spectrum of chitosan hydrogel was detected by spectrograph with halogen lamp as the light source; using the principle of light propagation on the surface of the medium and the principle of the polarization of light waves, the transmission spectrum of chitosan hydrogel is analyzed; the variation of the refractive index of chitosan hydrogel under different swelling degrees was studied, and it was found that the refractive power of gels with different wavelengths was different under different swelling degrees; through data processing, the range of gel refractive index change with the swelling degree and the sensitivity Q1 were calculated when the wavelength was 400 nm. By fitting the experimental data, the response law of refraction index of chitosan hydrogel to swelling degree and wavelength was obtained, and the fitting degree was higher, and it provides the experimental basis for the application of chitosan hydrogel in optical sensing. Through experiments, it is demonstrated that chitosan hydrogel has excellent swelling properties, wide range of refractive index variation and high sensitivity, it has certain advantages in intelligent wear, skin simulation, optical sensing and other fields.