Abstract: The hydrolytic and photolytic characteristics of pinoxaden under different conditions were investigated by high performance liquid chromatography (HPLC) and high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) in laboratory conditions. The results showed that in the buffer solutions of pH 4.0, 7.0 and 9.0, the hydrolysis half-lives of pinoxaden were 347, 40.8 and 1.08 h at 25 ℃, and were 57.8, 11.6 and 0.498 h at 50 ℃, respectively, which classified it as easily hydrolysis type. The hydrolysis of pinoxaden was rapid under alkaline conditions, but slowly under acidic conditions. The hydrolytic rate of pinoxaden increased with the increase of temperature, and the temperature effect coefficients ranged from 2.18 to 6.00. Under the irradiation of xenon simulated sunlight, the photo-degradation half-lives of pinoxaden in the buffer solutions were decreased with the increase of pH value, and the shortest was 10.0 h at pH of 8.0. The photolytic rates of pinoxaden in different natural water showed the following sequence: pond water > paddy water > river water > pure water, and the half-lives were 5.17, 7.79, 8.56 and 38.5 h, respectively. The main hydrolysis product of pinoxaden was M2 (8-(2,6-diethyl-4-methyl-phenyl)-9-hydroxy-1,2,4,5-tetrahydro-pyrazolo1,2-d1,4,5oxadiazepin-7-one), and the main degradation mechanism was the hydrolyzation of the ester group. M2 was further degraded under light conditions, indicating that photolysis was an important degradation way of pinoxaden. The results provided a scientific reference for the environmental behavior and safety assessment of pinoxaden in water environment.