Abstract: Quinclorac use causes significant losses in tobacco production. In order to clarify the mechanism of quinclorac phytotoxicity to tobacco, the ACS (1-aminocyclopropane-1-carboxylate synthase) family was identified and its expression pattern under quinclorac stress was analyzed. The results showed that there were 26 ACS genes in Nicotiana tabacum. The chromosomal location analysis suggested that 23 ACS genes were located on the chromosomes, and the other 3 genes were located on the scaffold. The subcellular localization prediction indicated that ACS genes were mainly localized in the nucleus or cytoplasm. Phylogenetic analysis showed ACS evolved into three types. The sequence alignment showed that amino acid sequences of ACS family contained conserved motifs and active sites. Cis-acting elements analysis revealed that NtACS genes contained 10 cis-acting elements, including abscisic acid, jasmonic acid, salicylic acid, auxin and gibberellin. Expression pattern analysis revealed that most ACS genes were up-regulated by quinclorac stress.