Abstract: The photolysis trend of acetochlor in different kinds of water under high pressure mercury lamp,xenon lamp and sunlight illuminating was investigated. The result showed that the photoreaction quantum yield at wavelength of 313 nm was 0.0692. Under the high pressure mercury lamp illuminating,the photolysis rate of acetochlor was in the order of pure water>river water>pond water>paddy water, and direct ratio to the pH of water solution. The photolysis rate of acetochlor in the pure water was in the order of high pressure mercury lamp>xenon lamp>sunlight illuminating