An amphiphilic macromolecule was first synthesized by conjugation of hydrophobic small molecule L-phenylalanine ethyl ester (L-phe) to the natural macromolecule γ-polyglutamic acid (γ-PGA). Avobenzone (AVB), isooctyl methoxycinnamate (OMC) and benzophenone-3 (BP-3) as organic sunscreen agents were then loaded into the γ-PGA nanoparticles through the macromolecular self-assembly for the preparation of sunscreen nanoparticles. The photostability, broad-spectrum sunscreen properties, skin permeability and cytotoxicity of these γ-PGA-based sunscreen nanoparticles were characterized. Compared with the unloaded organic sunscreen composition, the decrease in UVA absorbance of γ-PGA sunscreen nanoparticles was reduced from 12% to 3%, and decreased from 18% to 4% in UVB absorbance after UV irradiation, which proved the photostability improvement. At the same time, the γ-PGA sunscreen nanoparticles have broad-spectrum sun protection in the entire ultraviolet band and low skin permeability and low cytotoxicity. Moreover, the γ-PGA sunscreen nanoparticles were used as sunscreen agent in a typical sunscreen formulation, and the sun protection factors (SPF) of the sunscreen can reach to 75 at the 10% mass fraction loading.