In order to study the structure of mussels' foot and the characteristics of byssal thread formation (byssus secretion mechanism), the byssal secretion rate of of three mussels (Mytilus edulis, Septifer virgatus and Modiolus modiolus) were compared under different salinity (20, 22, 24, 26, 28 and 30).The histological structure and byssal secretion process of three mussels were discribed. In this study, the relationship between histological structure and byssal thread formation mechanism of three mussels were discussed.The results are as follows. ① three mussels began to secrete new byssal thread in 2 h after cutting off the old byssuses, three kinds of mussels showed significant differences in the ability of byssal thread formation within 24 h. The number of byssus was at the average of (28.45±6.14) for M. modiolus, and M. edulis and S. virgatus were (15.43±2.9) and (6.87±1.67), respectively; ② salinity caused apparent influences on byssal secretion of three mussels; M. edulis could secrete byssal thread under salinity from 20 to 30 and the byssal number was the highest in 22; S. virgatus stopped secreting byssal thread when salinity is higher than 28; ③ The histologic sections presented that there were microvillus covering the mussels foot and groove in the segmental venter which covered with fine cilia; the internal was composed of the glands, muscles and cavities. The glands were mucous glands, collagen glands, phenol glands, enzyme glands. The differences of the muscle distribution and the size of cavity of three mussel foot resulted in the density distinction of foot. Studies suggested that M. modiolus showed stronger ability of environmental adaptation than S. virgatus and M. edulis because of faster secretion of byssuses.