The swimming crab (), which is an economically important marine crab in China, is a widespread species in coastal Japan, Korea, and China. Neuropeptides have a significant effect on osmoregulation, growth, development, and immunity of crabs. To investigate the function of neuroparsin under low salinity stress in , the neuroparsin gene was cloned by rapid amplification of cDNA ends. The NP gene is 1920 bp long, including a 309 bp open reading frame (ORF) that encodes a 102-aa polypeptide; its isoelectric point was 7.42 and the molecular mass was 10.8 kDa. The NP gene contains 12 cysteine residues, which is a typical characteristic of neuroparsin in decapods. The homology and phylogenetic systematic analyses revealed that the highest homology and similarity (reaching up to 89%) occurred between P. trituberculatus clustered with . The tissue expression analysis showed that the expression of NP gene was relatively high in the brain, followed by the gill and eye, with very little or no expression in the ovaries, muscles, heart, liver, and pancreas. The expression pattern analysis of the NP gene under low salt stress condition showed that low salt stress can significantly change the expression of NP gene in the brain, gill, and eyestalk, and the overall expression was upregulated. In the brain, gill, and eyestalk, the expression of NP was 7.7, 2.8, and 2.6 times higher than that of the control, respectively (NP gene expression in the gills presented an increasing trend after the ablation of eyestalk. Furthermore, the expression of NP after the ablation of bilateral eyestalk was significantly higher than that after the ablation of unilateral eyestalk (NP gene might play a role in the salinity adaptation of , which is regulated by its neuroendocrine system.