L-amino acid oxidase (LAAO) is a kind of immune protease widely found in nature. In order to explore the gene sequence characteristics of LAAO (PmLAAO) and its expression changes after stimulation by Vibrio parahaemolyticus, the full-length cDNA of PmLAAO was cloned in this study. The length of its open reading frame (ORF) is 1767 bp, encoding a total of 588 amino acids, and has the structure domain of FAD-binding domain and amino acid oxidase (Amino_oxidase), which is a member of the amino acid oxidase family. The results of multiple sequence comparisons and phylogenetic trees showed that PmLAAO was closely related to bicrustacea, among which LAAO was the most similar to the Mytilus californianus. qRT-PCR results showed that PmLAAO was expressed in gill, mantle, adductor muscle, gonadal gland and digestive gland, with the highest expression level in gill tissue. After stimulation by Vp, the expression level of PmLAAO in gill tissue was significantly increased, and the expression level was the highest 48 h after stimulation. These results provide a reference for the functional activity of the novel immune protease LAAO in bivalve shellfish.