The study with high-pressure mercury lamp illuminating showed that after illuminated for 15 min, NO2- and NO3- quenched the photolysis of mefenacet, and NO3- with a concentration ratio 10:1 (mass) had the most obvious effect, its quenching rate being up to 53.3%. Halogen ions inhibited the photolysis of mefenacet by "weight atom effect". When the concentration ratio of I- was 10:1, the quenching rate was 76.9% after illuminated for 15 min. Surfactants Nongru 500, Nongru 404, Nongru 601 and Nongru 603 had different effects on the photodegradation of mefenacet. At concentration ratios 1:5 and 1:1, only Nongru 404 showed a weak photosensitive effect, while in the other cases, all the four surfactants had photoquenching effects. Among the four herbicides benthiocarb, bensulfuron, alachlor and chlorsulfuron, only bensulfuron at low concentration ratio (1 : 10) accelerated the photolysis of mefenacet, with a photosensitive proportion of 18.2% after illuminated for 25 min. Aerified N2 could accelerate the photolysis of mefenacet, and the half-life was shortened from 7.14 min to 6.70 min without aerifying N2.