Abstract: To investigate the adaptability of freshwater pearl mussels to saline–alkaline aquaculture conditions, Hyriopsis cumingii was selected as the experimental species. A one-year pilot culture was conducted in two saline–alkaline water areas, preliminarily confirming that H. cumingii can tolerant such environments. To further explore the adaptability of H. cumingii under different salinity stress, a progressive salinity elevation experiment (control, 0.5‰, 1‰, 2‰, 4‰, and 6‰) was carried out to analyze the effects of salinity stress on survival rate, antioxidant- and tissue-damage-related metabolic enzyme activities, hemolymph cell composition, osmotic pressure, and gill tissue morphology. The results showed that after one year of culture, there were no significant differences in shell width or shell length between the saline–alkaline groups (S1: salinity 3‰—3.5‰; S2: salinity 3.5‰—4‰) and the freshwater group (F1: salinity 0.15‰; P>0.05). However, shell height and body weight were significantly reduced in the S1 and S2 groups compared with the F1 group (P<0.05). The lightness L* value and total color difference △E* value of pearls produced in the S1 and S2 groups were significantly higher than those in the F1 group (P<0.05). Further results indicated that with increasing salinity, the survival rate of H. cumingii decreased significantly (P<0.05); at day 20, the survival rates in the 2‰, 4‰, and 6‰ groups were 100%, 86.2%, and 23.6%, respectively. Hemolymph osmotic pressure increased significantly with increasing salinity (P<0.05), and the proportion of granulocytes as well as their granularity complexity in the hemolymph increased significantly (P<0.05). The activity of Na⁺/K⁺-ATPase in the 4‰ group was significantly higher than that in the other salinity groups (P<0.05), but showed no significant difference compared with the control group (P>0.05). When salinity increased to 4‰ and 6‰, the length of lateral cilia in the gill tissue was significantly shortened (P< 0.05), and the width of gill water channels was significantly larger than that of the control group (P<0.05). In addition, the activities of superoxide dismutase (SOD) and catalase (CAT) exhibited an initial decrease followed by an increase with rising salinity, with significantly lower activities in the 2‰ and 4‰ groups than those in the control and 6‰ groups (P<0.05). In contrast, the activities of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) showed an initial increase followed by a decrease, reaching the highest levels in the 4‰ group and being significantly higher than those in the control group (P<0.05). Overall, salinity stress caused significant osmotic imbalance and immune damage in H. cumingii, while an active physiological response was observed at a salinity of 4‰, indicating that H. cumingii is able to regulate physiological status and adapt to salinity stress at this level. This study provides important data support for elucidating the salinity stress response of freshwater pearl mussels and for saline–alkaline aquaculture practices.