The cDNA of cyclin B gene was cloned from Hyriopsis cumingii by modern molecular biology techniques, and the sequence length is 1 024 bp, including 768 bp of ORF, 197 bp of 3'-UTR, encoding 255 Aa. Further analysis showed that cyclin B has a high similarity with oysters and scallops. Its amino acid sequence has a typical cyclin-box and two cyclin-dependent protein kinases, the site of action of CDK.RT-Q PCR showed that the expression of cyclin B was the highest in the gonads, and the expression level of cyclin B in females was significantly higher than that in males (P<0.05). The expression of cyclin B in other tissues was significantly lower than that in gonad (P<0.05), and there was no significant difference between male and female (P>0.05).After RNA interference (RNAi), the expression of gonad and gill was significantly decreased except for the mantle (P<0.05), indicating that RNAi has different silencing effects on different tissues of Hyriopsis cumingii. The phase changes of cells after RNAi were determined by flow cytometry. It was found that the proportion of G2/M phase in gonad and gill cells decreased, indicating that cell division can be regulated by cyclin B in Hyriopsis cumingii.The cyclin B gene and its preliminary functions were studied, which laid a molecular basis for further regulation of the cell growth in vitro.