Abstract: This study aims to investigate the tolerance of juvenile mandarin fish (Siniperca chuatsi) to alkalinity and the mechanisms by which alkalinity stress affects its physiological metabolism. Acute toxicity tests were conducted to assess the alkalinity tolerance of two sizes of mandarin fish (10 cm and 20 cm). A freshwater control group and high-alkalinity groups (15 mmol/L and 25 mmol/L) were established. Through acute stress experiments, indicators such as osmotic pressure, oxygen consumption rate, ammonia excretion rate, blood gas analysis, and free amino acid levels were measured. In terms of adaptability, the 20 cm group exhibited significantly higher alkalinity tolerance than the 10 cm group. Under alkalinity stress, the plasma osmotic pressure of mandarin fish was significantly elevated compared to the freshwater group and stabilized within 48 hours. As alkalinity increased, the oxygen consumption rate showed an upward trend, while the ammonia excretion rate initially decreased and then increased, with the lowest rate observed in the 25 mmol/L alkalinity group. Meanwhile, blood gas parameters in the alkalinity groups differed significantly from those in the freshwater control group (P < 0.05), with significantly higher plasma pH and HCO₃^- concentrations in the alkalinity groups. Regarding changes in free amino acids, the overall content of free amino acids in the plasma and muscle of mandarin fish showed an increasing trend under alkalinity stress, with significantly higher relative content in muscle than in plasma. However, free amino acid levels in plasma stabilized more rapidly within 48 hours, whereas those in muscle stabilized after 72 hours. mandarin fish possesses certain physiological regulatory capabilities in response to alkalinity stress, achieving osmotic and acid-base balance through mechanisms such as upregulating osmotic pressure, modulating energy metabolism, and accumulating free amino acids. This study clarifies the adaptability of juvenile mandarin fish to alkalinity, providing a theoretical basis for its seedling cultivation and domestication, as well as fundamental data and scientific support for optimizing saline-alkaline water aquaculture techniques.