The photo-transformation of microplastic (MP) in natural water may involve interactions with various ingredients, but the photoaging kinetics and underlying mechanism are not well understood. This work systematically explored the photoaging process of polystyrene microplastic (PS-MP) in the presence of commonly-found inorganic anions, including NO, HCO, Br and Cl. The addition of these ions led to more obvious changes in the morphology, functional groups and molecular weight of photoaging PS-MP. The evolution of carbonyl index value for the photoaged samples conformed to pseudo-first-order kinetic model, and the photoaging rate constant (k) in the presence of inorganic anions at their environmentally relevant concentrations of 0.6 mM, 1.2 mM, 0.1 M and 0.1 mM was calculated to be k = 0.0074 d, k = 0.01001 d, k = 0.00783 d, and k = 0.00888 d, which was higher than that in ultrapure water (k=0.00705 d). Electron paramagnetic resonance technique and quenching experiments demonstrated that photo-transformation of PS-MP was mainly mediated by indirect photolysis, i.e., the formation of reactive radical species. The photosensitivity of NO promoted more •OH production, thereby accelerated the indirect photoaging of PS-MP. Meanwhile, the presence of halide ions promoted the generation of reactive halogen species, which were also involved in the indirect photoaging of PS-MP. Interestingly, as •OH scavenger, HCO had no inhibitory effect on PS-MP photoaging, attributing to the oxidation of CO. This study provides valuable insights into the understanding of photo-transformation of MPs in natural aquatic environments.