Abstract: Long-term application of the herbicide quinclorac has led to high levels of resistance in barnyard grass (Echinochloa spp.). Amino acids play an important role in plant metabolic processes. By use of quinclorac-resistant barnyard grass E. crus-pavonis, this study examined the effects of external application of an equimolar mixture of L-methionine and L-cysteine (concentrations: 200, 400, 600, 800 μmol/L) on the growth, and antioxidant enzyme activity, and gene expression related to ethylene biosynthesis and cyanide detoxification in the plant exposed to 500 μmol/L quinclorac. In the absence of quinclorac, the treatment with mixtures of the two amino acids had no significant effect on growth, and did not show any effect on SOD and CAT enzyme activities either. The treatment of 600 and 800 μmol/L mixture significantly inhibited PPO enzyme activity. However, after exposure of the plants to quinclorac, the mixture stimulated the growth at lower concentration but inhibited the growth at higher concentration. Meanwhile, the activity of antioxidant enzymes SOD, CAT, PPO and detoxifying enzyme GSTs increased first and then decreased, while the activity of POD increased gradually with the concentration of L-methionine and L-cysteine elevated. More importantly, pretreatment of the two amino acid mixtures significantly activated the ethylene signaling pathway, and the expression of the ACC synthase gene, ACC oxidase gene, and β-cyanalanine synthase gene EC_CAS related to cyanide detoxification were increased. The mixture at 600 and 800 μmol/L strongly induced the expression of all five tested genes. This study revealed that exogenous application of L-methionine and L-cysteine regulates the resistance of barnyard grass to herbicide quinclorac by affecting the ethylene signaling pathway, which is of great significance in reducing the application amount of herbicides in rice fields and slowing down the resistance of barnyard grass to quinclorac.