Abstract: To clarify the phytotoxicity mechanism of quinclorac to tobacco, the isobaric-labeled relative and absolute quantitative (iTRAQ) technology was used to analyze the proteomic changes of tobacco malformed leaves compared with normal leaves after 7 days of quinclorac treatment. A total of 65 differentially expressed proteins were identified, of which 23 were up-regulated and 42 were down-regulated. They were involved in biological processes such as photosynthesis, defense/anti-stress, and cellular metabolites respectively, and had molecular functions such as binding, catalysis and transport. Fluorescent quantitative polymerase chain reaction (qPCR) results were consistent with the proteomics data. Two key genes in abscisic acid (ABA) synthesis pathway, ABA2 and the 9-cis-epoxycarotenoid dioxygenase NCED gene, responded to quinclorac stress signals, reached peak expression at 48 and 24h after treatment, and decreased to baseline level at 72h after treatment. The gene profile of up-regulated expression followed by rapid down regulation suggests that tobacco has a mechanism for resisting quinclorac stress. This study analyzed the expression changes of tobacco leaf proteins under quinclorac stress and obtained important differentially expresed proteins.