Sulfonamides are frequently detected in surface water and groundwater, which have the characteristics of low concentration, high toxicity, and being difficult to remove. In this study, an electro/peroxydisulfate system catalyzed with activated carbon (EC/AC/PS) was used to treat sulfamethoxazole (SMX). The results showed that the removal efficiency of SMX was 88.5% by the EC/AC/PS system. An increase of SMX concentration led to a decrease of SMX degradation efficiency, followed by an increase of intermediate products. Acidic conditions improved the degradation of SMX with optimum pH value of 5. A separate increase of AC concentration, PS concentration, and current density would enhance the degradation efficiency of SMX. But the extent was limited when reaching a certain level. There was an optimum plate spacing of 9 cm for SMX degradation efficiency. After repeating the use of AC 4 times, the removal efficiency of SMX still exceeded 80%. The free radical experiments showed that SO₄^•- played a leading role. The benzene ring structure of SMX was gradually decomposed with reaction. According to data analysis, the reaction kinetic model was C_t = C₀exp(-1.100×10^-3[AC]^0.4471[PS]^0.6397[current density]^0.5658[plate spacing]^0.8405t). This study implied that the EC/AC/PS process could effectively remove sulfonamide antibiotics in water, which was an environmentally friendly treatment method.