An in-situ refractive index method is applied to study the diffusion process of sucrose in hydrogels. Diffusion was monitored by recording the refraction of a laser beam when passing through a triangular hydrogel cell. The distribution of refractive index changes was obtained from the deviated distance of the linear beam and converted to the distribution of concentration. By employing Fick's second law, the diffusion behavior was investigated and the diffusion coefficient was obtained.