Abstract: Water salinity is an important factor in the reproductive migration of the adult Chinese mitten crab, Eriocheir sinensis. However, the physiological adaption of male E. sinensis to changes in salinity has been unclear. Here we investigated the osmoregulation and physiological metabolism of the adult male crabs at different salinities. The male crabs were exposed to salinities of 0, 6, 12 and 18 for a 60-day adaption. The results were shown below: (1) The serum osmolality and the concentrations of Na+, Mg2+ and Cl- increased significantly along with the elevation of water salinity (P0.05), but there was no significant difference in the serum K+ and Ca2+ between different treatments (P0.05). The serum osmolality of the all the treatment groups was significantly higher (P0.05) than that of the culture brackish water; (2) The posterior gills of the 0 salinity group exhibited significantly higher Na+/K+-ATPase activity than that of other treatments (P0.05), whereas no significant differences in the Na+/K+-ATPase activity were found between the 6, 12 and 18 salinity groups (P0.05); (3) Among the four treatments, the crabs from the 12 salinity group had the highest triglyceride (TG) level (P0.05) and the lowest uric acid (UA), glucose (Glu), malondialdehyde (MDA) and superoxide dismutase (SOD). Besides, there were no significant differences in the urea level and the alkaline phosphatase (ALP) activity in the serum between the salinity treatments (P0.05). (4) The level of MDA and the activity of -glutamyltranspeptidase (-GT) in the hepatopancreas were the lowest in the 6 group, and the activities of acid phosphatase (ACP) and -GT in the hepatopancreas were the highest in the 12 group (P0.05). These results suggested that compared to the freshwater, higher water salinity could cause the elevation in the haemolymph osmolality and the concentrations of the major ions in serum, as well as the decrease in Na+/K+-ATPase activity in the posterior gills of adult male E. sinensis. Most of the metabolism indices in the serum and hepatopancreas indicated that compared to other salinities, 12 reduced the metabolic level, and also enhanced the immunity and anti-oxidation activity in male E. sinensis.