The effects of Humic acid and Fulic acid on the photolysis of acetochlor in aqueous solution under the irradiation of high pressure lamp and Xenon lamp were studied.The results showed that both humic acid and fulic acid decreased photolysis rates of acetochlor, and displayed a strong photo quenching effect,the photo quenching effects of HA or FA on the acetochlor were significantly and positively related to the dosages of HA or FA,and there was a combined photo quenching effect between the HA and FA.