Abstract: A 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 determined to be 4.0 mg/mL by the UV standard curve method. Surface tension, fluorescence, dynamic light scattering and Zeta potential were used to study the aggregation properties of CA-HA on the surface of an aqueous solution and in the solution. CA-HA can reduce the surface tension of the aqueous solution to 32 mN/m, forming aggregates with a particle size of 236 nm and a Zeta potential of −44.8 mV in aqueous solution. After UV irradiation, compared to CA alone, the UV absorbance of CA-HA at 250—325 nm doesn't show a significant blue shift, which suggests that the side chain of HA can inhibit the photoisomerization of CA probably through a steric effect. In addition to the fluorescence of monomeric CA, CA-HA exhibits fluorescence emission over a spectral range of 350—500 nm, characteristic of dimeric CAs. A significantly enhanced aggregation-induced fluorescence emission is observed when the concentration of CA-HA increases. A CA-HA emulsion is shown to have good sunscreen performance in the UVB band (275—320 nm).