Previous studies have shown that p21 activated protein kinase 2 (PAK2) plays an important role in promoting cell proliferation and anti-apoptosis in organisms, and can activate the expression of genes related to cell proliferation and growth. In order to explore the role of p21 activated protein kinase 2 (PAK2) gene in the proliferation and metabolism of histocyte of Onchidium reevesii, the cDNA of O.r- PAK2 gene was cloned by RACE technology, and the corresponding tissue expression was analyzed by real-time quantitative PCR(qRT-PCR). The results showed that the total length of cDNA of PAK2 was 1 809 base pairs(bp), comprising a 183 bp 5’ untranslated region (UTR), a 138 bp 3’UTR, and 1 488 bp open reading frame (ORF)which encodes 495 amino acids. Based on the homology analysis of amino acid sequences, the similarity of PAK2 gene between O. reevesii and Biomphalaria glabrata was up to 83%, with a homology of 88%. O. reevesii is related to the evolution of Elysia chlorotica. qRT-PCR results showed that PAK2 gene was expressed in all tissues. The expression level of intestinal tract and back was significantly higher than that of other tissues, followed by nerve tissue and hepatopancreas. The expression level of protein gland was the lowest, and the expression level of abdominal foot and mouth organ was at the medium level. In this study, the bioinformatics characteristics and expression levels of PAK2 gene in various tissues were preliminarily clarified. The study laid the foundation for specific action mechanism of shellfish.