To investigate the adaptation mechanisms of sea cucumbers to low salinity environments, physiological indices were measured in the coelomic fluid of Apostichopus japonicus under acute low salinity stress. Test specimens weighed (16.93±3.08) g (±SE) in body weight. Experiments simulated the salinity changes observed in culture ponds in the rainy season. Salinity was gradually lowered from 30 to 18 ppt at a rate of 0.5 ppt/h, where it was maintained for 96 h. Salinity was then increased back to 30 at the same rate, where it was maintained for 24 h. The osmotic pressure of the coelomic fluid was measured directly with a cryoscopic osmometer. Other physiological indices were measured using the corresponding test kits. The results showed that trends of variation in the osmotic pressure, total protein concentration, and Na⁺, K⁺, and Cl⁻ concentrations of the coelomic fluid were consistent with the changes in salinity. The close relationship of osmotic pressure and water salinity indicate that sea cucumbers osmoregulate their coelomic fluid to approximate levels of environmental salinity. Minimum concentrations of Na⁺, K⁺ and Cl⁻ (131.15±14.42) mmol/L, (6.0±0.24) mmol/L, and (141.76±2.13) mmol/L, respectively] occurred at 18 ppt. After salinity was returned to normal levels (30 ppt), the three indices showed no significant difference from the control group (P<0.05). Experimental Ca²⁺ concentrations were maintained at higher levels than in the control group, which varied from (3.78±0.49) mmol/L to (6.28±3.69) mmol/L (mean±SE). The Na⁺-K⁺-ATPase activity of the coelomic fluid over 4 d at 18 ppt was significantly higher than at other salinities (P<0.05), indicating activation of Na⁺-K⁺-ATPase activity under salinities of 18. This result demonstrated that the osmoregulatory system of the sea cucumber was activated in external environments of low salinity. Variation in Na⁺-K⁺-ATPase activity can alter membrane permeability to maintain the balance of osmotic pressure between the extra and intracellular environments. Compared with the control group, the total protein concentration in the coelomic fluid initially decreased, followed by an increase to a steady value that was then maintained for the duration of the experimental period. Glutamate pyruvate transaminase activity of the coelomic fluid, which can indirectly reflect the condition of protein metabolism, showed no significant change under differing salinities. This result indicated that changes in salinity had little effect on the respiratory metabolism of the sea cucumber and that sea cucumbers can adapt to changes in the external salinity. Osmolarity of the coelomic fluid was strongly dependent on the concentrations of Na⁺, K⁺ and Cl⁻. This experimental work on the adaptation of sea cucumbers to environmental salinity furthers understanding of sea cucumber physiology and ecology and will inform future work on the aquaculture of sea cucumbers.