High seawater temperatures in summer have a significant effect on the biochemical responses of the Pacific oyster, , and influence the survival rate of cultured oysters. In this study, we investigated the high temperature tolerance of ‘Haida No. 3’ through exposure to acute and gradually increasing temperature stresses over 72 h, and measured changes in five immune indicators:catalase (CAT), superoxide dismutase (SOD), lysozyme (LSZ), total antioxidant capacity (T-AOC), and malondialdehyde (MDA). In the gradual temperature increase test, we increased the seawater temperature progressively (2℃/d). In the acute seawater temperature increase test, we transferred oysters from rearing temperature (16℃) to 18℃, 22℃, 26℃, 30℃, and 34℃ directly. The results showed that when the water temperature was increased gradually, the survival temperature maximum (STMax) of ‘Haida No. 3’ was 33.63℃, that the critical temperature maximum (CTMax) was 40.13℃, and that the 50% critical temperature maximum (50%CTMax) was 36.67℃. In the acute water temperature increase test, the median lethal temperature after 72 h (72-h LT₅₀) was 30.13℃. In each treatment, the five immune indicators in the visceral mass changed significantly as exposure time increased. Within the first 12 h, significant increases were observed in the activities of CAT, SOD, LSZ, and T-AOC, after which their activities returned to their initial levels. The content of MDA was at its highest level between 6 h and 9 h, after which it decreased gradually. All of these results indicate that high-temperature stress induces significant changes in both the antioxidant immune response and in the activity of lysozyme in ‘Haida No. 3’ and greatly influences its survival. The high temperature tolerance noted in this study will provide a reference for the application and promotion of Pacific oyster strain ‘Haida No. 3’.