Abstract: To investigate the effects of common inorganic ions on the degradation of the anticancer drug capecitabine (CAPE) by UV/peroxymonosulfate (UV/PMS) process, the influences of Cl^–, HCO₃^–, SO₄^2– and NH₄⁺ at different concentrations were examined. Results showed that all four ions exhibited inhibitory effects on CAPE degradation, with inhibition intensity ranked as NH₄⁺ ＞ HCO₃^– ＞ Cl^– ＞ SO₄^2–. When the ion concentration increased from 0 to 10 mmol/L, both Cl^– and HCO₃^– caused a significant decrease in the degradation rate constant (k_obs). The inhibitory effect of SO₄^2– was weak, and the k_obs remained essentially stable. NH₄⁺ exhibited the most pronounced inhibition, with a reduction in k_obs reaching 46.7%. The results reveal the differential inhibition mechanisms of various water ions on CAPE degradation and provide a theoretical basis for the application of UV/PMS process in complex water environments.