Abstract: In the biochemical metabolic processes of Sophora alopecuroides, a lysine decarboxylase (LDC) gene is one of the key enzyme genes involved in the process of Oxymatrine biosynthesis. In the present study, the full length of the LDC coding sequence in S. alopecuroides was cloned using a pair of specific primers designed based on the LDC sequence of Sophora flavescens and was named Sa-LDC (gene bank accession number: KM249871). Sa-LDC belongs to the Type Ⅲ Pyridoxal 5-phosphate (PLP)-Dependent enzyme supergene family, is comprised of a 1368 bps open reading frame (ORF) without intron, and has 97% identity with the LDC of Echinosophora koreensis and S. flavescens in GeneBank. Its nucleotide sequence encodes 455 amino acid residues whose putative protein had a relative molecular mass of 49.14 kD and the theoretical isoelectric point of 5.63 without signal peptide and transmembrane structure. Interestingly, the deduced amino acid sequence of Sa-LDC had the conserved amino acid residue (Phe³⁴⁰) in quinolizidine alkaloid producing plants. Therefore, the S. alopecuroides and other quinolizidine alkaloid producing plants were placed into a single group in the phylogenetic tree. In addition, the real time fluorescence quantitative PCR (qPCR) and high performance liquid chromatography (HPLC) results showed that both the LDC expression level and oxymatrine content were influenced by polyethylene glycol (PEG) stress, and that LDC expression and oxymatrine accumulation in S. alopecuroides were correlated.