In this study, we used rapid amplification cDNA ends (RACE) methods to obtain the full-length cDNA of Myostatin gene in Onchidium struma. The bioinformatic analysis and expression pattern of MSTN mRNA in different tissues detected by Real-time fluorescent quantitative PCR (qRT-PCR) were performed. The full length of MSTN cDNA sequence consists of 2667 base pairs (bp), comprising a 374 bp 5′ untranslated region (UTR), a 643 bp 3′UTR, and a1650 bp open reading frame (ORF) which encodes 549 amino acids. The MSTN protein was predicted to contain 8776 atoms and its formula is C₂₇₇₄H₄₃₃₁N₇₈₃O₈₆₂S₂₆, with a calculated relative molecular weight of 63.27 kD and pI of 6.02. The result of signal peptide prediction shows that the N-terminal has a signal peptide of 21 amino acids length. The common features were found in MSTN of O. struma, including a conservative hydrolytic site (RSRR) and 9 cysteine residues. The phylogeny analysis shows that the relationship between O. struma and Aplysia californica was the closest. According to RT-PCR results, MSTN gene was expressed in various tissues with the highest expression in the hepatopancreas and lowest in foot; expression in muscle is lower than that in internal organs, and expression in muscle of abdominal skin is higher than other parts. Our results suggest that MSTN gene may play an important role in muscle growth and development in O. struma, and may provide useful information for further studies on evolution of marine invertebrates from sea to wetland.