Euryhaline fish, which can survive and grow in a wide range of salinity environment, have a complex mechanism to deal with salinity change. It is generally believed that at the isotonic environment, the energy consumption of fish used for osmotic regulation is the least, which was beneficial to fish growth. No previous study has reported about the isotonic point in Lateolabrax maculatus. Therefore, in this study, the blood osmotic pressure, Na⁺/K⁺/Cl^- concentration, Na⁺-K⁺-ATPase activity and relative gene expressions were detected during the periods of water desalination and freshwater adaptation. Result showed that the isotonic point of L. maculatus was 11.4 according to the changing characteristics of the osmotic pressure in fish plasma and the corresponding osmotic pressure of different water salinities. During the experiment of water desalination, both the concentrations of Na⁺, K⁺ and Cl^-and serum osmotic pressure were decreased significantly, with a slow recovery after freshwater adaptation. The results of mRNA expression of NKA genes in gill showed that the change trend of NKAα1a and NKAα1b was similar to Na⁺-K⁺-ATPase activity, except that the expression of NKAα1a in fresh water was not significantly different from that of the 30 seawater group, but the expression level of NKAα1b in fresh water was lower than 30 seawater group. After 1 day's water desalination, the expression level of NKAα3 decreased significantly, and remained low expression during fresh water adaptation. This study provided theoretical basis for desalination culture of L. maculatus.