Abstract: Hydrophobically modified hyaluronic acid (CA-HA) was prepared by the esterification reaction of the carboxyl group of cinnamic acid (CA) with the hydroxyl group of hyaluronic acid (HA). The maximum solubility of CA-HA in the aqueous solution was 4 mg/mL, which was measured by the UV standard curve method. The surface tension, pyrene fluorescence probe, dynamic light scattering and Zeta potential methods were used to study the aggregation properties of CA-HA on the surface of the aqueous solution and in the solution. It was measured that CA-HA can reduce the surface tension of aqueous solution to 32 mN/m, and form the aggregates with the particle size of 236 nm and Zeta potential of −44.8 mV in aqueous solution. After UV irradiation, compared with the non-grafted CA, the UV absorbance of CA-HA at 250—325 nm did not show a significant blue shift. This showed that the long chain of HA with the sugar ring unit can increase the steric hindrance, thus inhibiting the photoisomerization of CA graft group in CA-HA, and keeping the CA graft group in the trans configuration. In addition to the fluorescence emission peak of CA graft group, CA-HA had a fluorescence emission peak at 370—500 nm, which was resulted from the formation of CA graft group dimers. As CA-HA concentration increased, CA-HA can produce significantly enhanced aggregation induced fluorescence. At the same time, CA-HA emulsion had good sunscreen performance in UVB band (275—320 nm).