Abstract: The photodegradation behavior of the mixture of chlorothalonil and profenofos in water was investigated under high pressure mercury lamp (HPML). The results showed that the half-life of chlorothalonil at 3 μmol/L was 122.46 min under the HPML irradiation, and the half-lives of chlorothalonil were 64.54, 38.25 and 33.05 min in the presence of 1, 2 and 3 equivalent of profenofos. The half-lives of 3, 6, and 9 μmol/L profenofos were 87.85, 86.11 and 87.63 min, while they were 85.57, 83.51 and 84.32 min in the presence of chlorothalonil, respectively. This indicates that profenofos promoted the photodegradation of chlorothalonil in water and the promotion effect was increased as the concentration of profenofos increased. Chlorothalonil did not affect the photodegradation of profenofos. The main photodegradation product of chlorothalonil was 4-hydroxyl chlorothalonil with and without profennofos. Five photodegradation products of profenofos were detected, namely, 4-bromo-2-chlorophenyl hydrogen phosphate ethyl ester, O-(4-bromo-2-chlorophenyl)-O-ethyl-S-hydrogen phosphate, O-(2-chlorophenyl)-O-ethyl-S-propyl phosphorothioate, (2-chloro-4-hydroxyphenyl)-O-ethyl-S-propyl phosphorothioate, and 2-chloro-4-bromophenol. Profenofos promoted the degradation of chlotothalonil via the generation of more hydroxyl radicals. This study provides important theoretical support for the evaluation of the environmental ecotoxicity of combined pollution of chlorothalonil and profenofos.