Abstract: Oysters are currently one of the most economically important marine bivalves in China. Hong Kong oyster (Crassostrea hongkongensis), a dominant cultured oyster species along the southern coast of China, serves as a crucial high-quality protein source in the national diet. Salinity is a key environmental factor affecting its vitality and survival rate. The summer mass mortality phenomenon in bivalves is not only linked to pathogen infections (e.g., viruses and bacteria) but also closely associated with abiotic factors such as environmental stressors. With the rapid development of coastal economies, industrial and agricultural wastewater pollution in estuaries and coastal areas has intensified. Both salinity fluctuations and heavy metal contamination may significantly impact oyster survival. Among common estuarine pollutants, copper (Cu), including its ionic forms (Cu⁺ and Cu²⁺), is a major concern. However, limited research has been conducted on the tissue responses and antioxidant stress-related enzyme activities of C. hongkongensis under dual stress from Cu²⁺ and salinity. This study aims to investigate the salinity adaptability of C. hongkongensis in seawater environments containing Cu²⁺, with the goal of providing foundational insights for developing new oyster varieties with broader salinity tolerance thresholds. This study investigates the salinity adaptability of C. hongkongensis under Cu²⁺-contaminated marine environments. A 30-day dual-stress experiment combining salinity and Cu²⁺ exposure was designed. The activities of glutathione-S transferase (GSH-ST) and malondialdehyde (MDA) enzyme were measured, and the gill and myocardial damage were observed by sectioning. The result revealed that the final survival rate of C. hongkongensis decreased with increasing salinity and Cu²⁺ concentration. At 0.01 mg/L Cu²⁺ concentration and salinity of 5, 15, 30, the survival rates at 30 d were 100%, 100%, 87%, respectively. At 0.01 mg/L Cu²⁺ and salinity of 5, 15, 30, the survival rates at 30 d were 100%, 70% and 80%, respectively. At 1 mg/L Cu²⁺ concentration and salinity of 5, 15, 30, the survival rates at 30 days were 80%, 57%, 37%, respectively. The result indicate a synergistic toxic effect of high salinity and Cu²⁺. Under salinity 30 conditions, the 1.00 mg/L Cu²⁺ exposure group exhibited discoloration in soft tissues by day 13, along with abnormal lesions in gill filaments and myocardial tissues. Significant differences (P < 0.05) in GSH-ST and MDA levels were observed across time points, suggesting that the oxidative stress defense mechanism of C. hongkongensis under dual salinity-Cu²⁺ stress may progress through resistance, damage, recovery, and long-term adaptation phases. These findings provide a theoretical basis for developing new oyster varieties with broad salinity adaptation thresholds.