In this study, a static toxicology experiment was conducted to analyze the survival rate, Na ⁺/K⁺-ATPase (NKA) activity and phagocytic ability of the hemolymph of Sinonovacula constricta of two different sizes (small-size SSc and large-size LSc) under the acute stress of high pH and carbonate alkalinity (C_A). The results showed that when the C_A concentration was 2.5 mmol/L and the pH was 7.5-9.5, the survival rates of the two sizes were close to 100%.When the pH value was greater than 9.5, the survival rates of the two sizes were significantly decreased.When the concentration of C_A was 0-44.58 mmol/L and the pH value was 9.0-10.0, the survival rate of clams decreased with the increase of C_A concentration. Under the condition of pH 9.5, the NKA activity of LSc gill tissue increased with the increase of C_A concentration, and the phagocytic capacity of LSc haemolymph decreased with the increase of C_A concentration. It can be seen that S. constricta showed strong tolerance to high pH or C_A, but high pH and C_A had a great effect on the survival rate of S. constricta. The research results provide a theoretical reference for further exploration of breeding of S. constricta in saline-alkali soil.