The salinity will fall depending on the climate in the experimental aquaculture areas of ). And these ares were polluted to a certain extent by some heavy metal such as copper. Therefore, it is necessary to investigate the effect of different CuSO₄ concentrations on silver pomfret in low salinity sea. This study investigated the effects of low salinity on juvenile silver pomfret firstly. We decreased salinity from 24 to 12 by three steps, the salinity change of every step was 4. When fish was in a stable condition, they concentrations was set to in salinity 12, or 0, 0.5 mg in salinity 24. The copper expose was sustained for 144h. Then two kinds of ion-regulatory enzyme activities in gill: Na⁺/K⁺-ATPase (NKA) and V-H⁺-ATPase (VHA) activities, three kinds of antioxidants in liver: glutathione (GSH) contents, superoxide dismutase (SOD) and catalase (CAT) activities of silver pomfret were tested in order to reveal the the effect of water copper on those index of silver pomfret in low salinity sea. In this study, NKA and VHA activities rose then decreased during the salinity decline. After CuSO₄ added into water4 concentration. VHA activities decreased significantly after CuSO₄ added, especially group at salinity 24 increased at 24th hour and then decreased. GSH contents and SOD activity increased significantly through the salinity decline. Meanwhile, CAT activity fluctuated. After CuSO₄ added into water· groups decreased firstly, then increased significantly at 72 h. SOD activities of each CuSO₄ group increased firstly then returned. CAT activities of 0.3 and 0.5 mg increased significantly at 72 h. This study suggested that water copper affected silver pomfret with inhibition of NKA and VHA, especially, in low salinity. The change of GSH, SOD and CAT can reflect the damage of low salinity and CuSO₄ on silver pomfret. The resistibility of silver pomfret to water copper is between a certain range, but we should pay attention to the concentrations of heavy metal such as copper in water when the water environment has changed, for example, salinity had fallen.