Pharmaceuticals and personal care products (PPCPs) in drinking water have attracted increasing attention from domestic and international scholars, and Trimethoprim (TMP) is one of the common drugs in the aqueous environment. Since reactive substances such as HO• and Cl• and ClO• are generated in the UV/free chlorine oxidation system, it is considered as an emerging alternative to the UV/H₂O₂ process. However, since the background conditions of the water body and the oxidant concentration have an effect on the oxidation effect, a comparison of the UV/free chlorine and UV/H₂O₂ oxidation systems under different conditions was carried out in this paper. It was found that the removal efficiency of TMP by UV/free chlorine was 16% higher than that by UV/H₂O₂ at an oxidant concentration of 0.05mM, and the removal efficiency of TMP degradation by UV/free chlorine was 17% higher than that by UV/H₂O₂ at a HCO₃^-/CO₃^2- concentration of 1mM. The removal efficiency of TMP by UV/free chlorine was 17% higher than that by UV/H₂O₂. The UV/free chlorine oxidation system was superior to the UV/H₂O₂ oxidation system in degrading TMP. The main reason for this phenomenon is that the quantum yields of HClO and OCl- are higher than those of H₂O₂. Under the same UV intensity and concentration conditions, UV/free chlorine oxidation system can produce more free radicals in the same time, so UV/free chlorine oxidation effect is better and less costly.