To further understand the aging behavior of microplastics in nature influenced by environmental factors, we investigate the aging mechanism of micron-sized PS particles at a single-particle level by simulating UV radiation. The spectral characteristics of PS particles under particle size and aging time are characterized using the single-particle Raman spectroscopy, and the principal component analysis further reveals the changes in the chemical structures of PS particles under UV exposure. UV radiation is found to damage the structures of PS polymers and PS molecules, and the small particle size, which is more significantly damaged, is degraded to a higher extent. These results suggest that the microplastics with different sizes exhibit different behavioral processes when facing with the same environmental factors, and that small size microplastics are more likely to be decomposed into small particles or small fragments that are potentially more threatening after UV radiation. This study provides new insights into the aging dynamics of microplastics in the environment from the single particle level.