During the production of desalination plants, concentrated seawater is expelled to local marine environments, which may lead to hypersaline pollution. This study aimed to investigate the effects of concentrated seawater on Porphyra haitanensis by exposing P. haitanensis to salinity of 40‰, 36‰ and 30‰ (considered as the control) for 2 h, 6 h, 12 h, 24 h, 48 h and 96 h. The results showed that chlorophyll a content showed no significant change among different salinities, while carotenoid, phycocyanin and phycoerythrin contents increased in treatment with 40‰ compared with the control. Carotenoid, phycocyanin and phycoerythrin can all function as antioxidants, suggesting that hypersaline exposure might trigger oxidative stress in P. haitanensis. Overall, the present study suggests that hypersaline pollution caused by desalination plants raises significant ecological risks for local marine environments.