Changes in plasma osmolality, concentration of ions (Na+, Cl–, K+) and gill Na+/K+–ATPase activity of goby (Synechogobius ommaturus Richardson) young fish were valued by gradually changing the medium salinity from 6 (control) to 0, 5, 10, 15, 20, 25, 30, 35, 40, and 50. The changes of both of the plasma osmolality and ions showed a similar trend, which increased with the increasing of medium salinity when the salinity was in the range of 0–50; the plasma osmolality and ions were significantly higher compared to the control group (salinity 6) and remained steady at salinity of 0–35. Na+/K+–ATPase activity reached its peak at the salinity of 0. Na+/K+–ATPase activities of goby young fishes in groups with salinity at 25 and 35 were evidently stronger than those in groups with salinity range of 5–20 and of 40–50. There were no significant differences in Na+/K+–ATPase activities between 25 and 35 groups and the differences were also of no statistical significance in groups with salinity range of 5–20 and of 40–50. In salinity range of 5–35, the gill Na+/K+–ATPase activity increased with the ascending of the degree of salinity. Regression analysis showed the serum isosmotic point was 0.308 mol/kg, which corresponded to salinity 10.8. The isoionic point of Na+, Cl– and K+ were 340.28 mmol/L, 137.08 mmol/L and 4.51 mmol/L, respectively, which corresponded to salinity 28.1, 8.9 and 18.5. The results indicated that the goby young fish might have a great capability of acclimation to salinity change. It tolerates salinities ranging from 0.3 to 35, and the optimal living salinities were in the range of 5 to 15.