Abstract: In order to determine the residual threshold value of quinclorac in tobacco-rice rotation soil, the residue and degradation of quinclorac in the soil under different dosages were studied, and the correlation between the residue of quinclorac and the phytotoxicity level of tobacco was analyzed. The results showed that low concentration of quinclorac residues in soil could lead to tobacco damage. The half-life of quinclorac recommended in rice field was 25.7 days, and the degradation rate was 77.83% after 60 days. At 60 days after transplanting, there was a good exponential relationship between the residual amount of quinclorac and the phytotoxicity level of tobacco, and the residual threshold was 0.006 mg/kg. If the residual threshold was exceeded, tobacco would have obvious phytotoxicity symptoms. During the planning period of tobacco field, i.e. 60 days before transplanting, the residual risk value of quinclorac was 0.072 mg/kg. The research provided theoretical basis and data support for controlling the risk of quinclorac and tobacco field planning.