To explore the effects of salinity on the Na /K -ATPase activity, the osmolality of pericardial cavity fluid and peritoneal fluid and ion content (Na , K and Cl-) in Onchidium struma, six hundred healthy specimens (wet weight 10.24 ? 2.16 g) were selected and had been raised for 30 days with simulated ecological environments that in different salinity conditions (5, 15, 25, 35 and 45). The results shows: 1) The salinity and osmolality of peritoneal fluid and pericardial cavity fluid of Onchidium struma are increased with increasing ambient salinity, and the increased amplitude of peritoneal fluid all greater than the pericardial cavity fluid. 2) The content of Na , Cl- and K of pericardial cavity fluid consistent with the changes of ambient salinity and these are all increased firstly then decreased. The concentrations of Na , Cl- and K reached the maximum at salinity 25 and salinity has a significant impact on Na and Cl- but with no significant effect on K . 3) There are significant differences on Na /K -ATPase activity of different parts in Onchidium struma. The Na /K -ATPase in cardia stomach, intestine, epidermis, muscle and pericardial cavity film of Onchidium struma have higher activity. In addition, the Na /K -ATPase activity of different parts activated by salinity with some tissue-specificity, there are higher activity of hepatopancreas, cardia stomach and epidermis in low salinity while the pyloric stomach, intestine, kidney and pericardial cavities film in high salinity. These results indicates that the salinity can significantly impact the osmolality and ion concentration of peritoneal fluid and pericardial cavity fluid as well as the Na /K -ATPase activity of Onchidium struma. The cardia stomach, intestine and muscle are main organs involved in its ion transport and osmoregulation with strong osmotic adjustment capability.