【Objective】This study aimed to investigate the role of mitochondrial β-cyanoalanine synthase (CAS) in the induction of the ethylene (ETH)-associated alternative oxidase (AOX) pathway in response to salt stress in woody plants. 【Method】 We studied the 1-aminocyclopropane-1-carboxylic acid (ACC, an ETH precursor) content using high performance liquid chromatography, the gene relative expression via quantitative real-time PCR, and cysteine levels using colorimetric methods in NaCl-treated Bopulus×enramericana ‘Nanlin 895’ (salt-insensitive clone hybrids) leaves. 【Result】 Salt stress to ‘Nanlin 895’steckling caused an accumulation of ACC and rapid up-regulation in the expression of ETH biosynthetic genes ( ACS7 and ACO3), the mitochondrial CAS gene (CYS C1), and the β-cyanoalaninenitrilases gene (NIT 4), followed by an increase in AOX1b expression and a decrease in the transcription level of the cytochrome c oxidase gene (COX6b) in the leaves. The application of salicylhydroxamic acid (SHAM, an AOX inhibitor) significantly reduced AOX1b expression and elevated the malonyldialdehyde (MDA) concentration and electrolyte leakage (EL) level but had no evident effect on the expression of CYS C1. Conversely, the modulation of aminooxyacetic acid (AOA, an ETH biosynthesis inhibitor) not only reduced CYS C1 transcription, but also blocked salt-induced AOX1b expression and increased the levels of MDA and EL. Furthermore, AOA recovered the salt-reduced cysteine content, whereas both SHAM and antimycin A (an AOX activator) failed to affect the cysteine level. 【Conclusion】 These results indicate that mitochondrial CYS C1 is involved in the ETH-induced pathway, functioning upstream of AOX in poplar responses to salt stress.