Abstract: Scylla paramamosain is one of the most important cultured commercial crabs on the southeast coast of China. DDX5 gene is a member of the DEAD-box helicase gene family. Previous studies showed that it played an important regulatory role in the maturation, differentiation and function maintenance of germ cells. SpDDX5 gene sequence from the transcriptome database of S. paramamosain was obtained and its expression patterns in various tissues, testicular development and embryo development were examined by qRT-PCR. In addition, the possible role of SpDDX5 in gonadal development was analyzed by the RNAi technique. Results show that the open reading frame of SpDDX5 is 1 893 bp in length, which encodes 630 amino acids and has two domains of DEXDc and HELICc. SpDDX5 was widely expressed in various tissues and the expression level of SpDDX5 in the testis was significantly higher than that of other tissues (P<0.05). During testicular development the expression level of SpDDX5 decreased gradually. During embryonic development the expression level of SpDDX5 gradually increased. After SpDDX5 interference, the expression level of Vasa and Nanos, the molecular marker genes, was up-regulated. The expression level of gonadal development-related foxl-2gene was down-regulated. The expression level of Dsx was up-regulated and Dmrt-like and idmrt-2, the genes from Dmrt family, were down-regulated. According to the results, SpDDX5 gene may play an important role in the development of germ cells and testes of S. paramamosain.