In the present study, full length cDNA, encoding the ), was cloned using 3' Race and 5' Race techniques, and the sequence and structural analysis of was conducted using bioinformatics methods. The results showed that the full-length cDNA encoding consisted of 2021 bp nucleic acids in length, including a 5'-UTR of 36 bp, a 3'-UTR of 656 bp and an open reading frame (ORF) which encoded 442 amino acids. Analysis of the protein domain features showed that the deduced polypeptides contained two conservative domains characteristic of MH1 and MH2. Multiple sequence alignment revealed that the amino acid sequences of EsSmad3 have the 0.679, 0.691, and 0.619 identity with respectively. The tissue distribution of mRNA in sexually mature individuals and different muscle groups during the molt cycle in juvenile crabs, were analyzed using quantitative real-time PCR (qRT-PCR). In sexually mature crabs, the transcript was detected in the eyestalk, claw muscle, ovary, heart, hepatopancreas, epidermis, testicle, gill, and triangular membrane, and the expression level was relatively high in the eyestalk and testicle, and was low in the hepatopancreas and heart. In juvenile crabs, the transcript in different muscle groups was different depending on the molt stage. In walking leg muscles, the expression level was higher in inter-molt C stage than in the later pre-molt D_3-4 and post-molt A-B stages, but there was no statistically significant difference ( expression level decreased rapidly in the pre-molt D_3-4 stage (<0.05) and increased in the post-molt A-B stage, lasting to the inter-molt C stage ( expression level was much higher in the inter-molt C stage than in the post-molt A-B stage (stage. These results suggest that the expression of transcript in different muscle groups was related to the molt stage of is involved in muscle atrophy, growth, and rebuilding during the molt cycle of