Abstract: In order to explore residual contamination risk of dimetachlone,the decline dynamics of dimetachlone in tobacco and soil was investigated under field-simulated condition and field using gas chromatography.The results showed that the decline rate of dimetachlone in sterilized soil was 3.5 times slower than in unsterilized soil,indicating that microorganism was a dominant factor affecting decline of dimetachlone in soil.Some factors including soil properties,temperature,water holding capacity,addition concentrations could also affected decline of dimetachlone in soil.The decline rate of dimetachlone in five kinds of tobacco-planted soils showed the following sequence:Huaxi soil>Qiannan soil>Kaili soil>Zunyi soil>Bijie soil.The decline rate of dimetachlone in soil increased with temperature and water holding capacity,but decreased with application doses.In the field,the half-life of dimetachlone in tobacco leaves and soil was 5.8 and 8.2 d,respectively.