The degradation of atrazine (ATZ) by ultraviolet/peroxymonosulfate (UV/PMS) under different conditions was investigated in phosphate buffer, and the degradation mechanism and kinetics were discussed. The results showed that the degradation rate of 2.5 μmol/L ATZ in UV/PMS system was 97.63% in 20 min when the reaction temperature was 20ºC, the concentration of PMS was 20 μmol/L and the UV intensity was 50 mW/cm² in pH7 phosphate buffer. The mechanism analysis showed that PB with partial alkalinity promoted the degradation of ATZ by UV/PMS more than that with acidic PB. The effect of PB with alkaline conditions on the degradation of ATZ by UV/PMS was more complicated and mainly related to the state of phosphate ions. The UV/PMS system contained both HO• and SO₄^-•, and the ratio of HO•, SO₄^-•and UV-degraded ATZ was nearly 1:1 in pH7 PB. Inorganic anions experiments showed that Cl^- and HCO₃^- inhibited the degradation of ATZ under UV/PMS, and the inhibitory effect of Cl^- was more obvious. NO₃^- promoted the degradation of ATZ by UV/PMS. Kinetic analysis showed that UV/PMS degradation of ATZ reaction kinetics was more in line with the quasi first-order reaction kinetics, the inhibition effect of the same concentration of ETA and Clon UV/PMS degradation of ATZ are the same, and UV/PMS degradation of ATZ decreased by 38.54% and 36.29% respectively. The addition of NO₃^- increased the rate of degradation of ATZ by UV/PMS by 31.21%. By LC-MS analysis, 5 kinds of production m/z and 6 kinds of products were obtained.