Abstract: Excess carbon dioxide in atmosphere is absorbed by seawater and then form carbonic acid, which increases seawater acidity. To investigate the effects of seawater acidification on antioxidant and immunity in juvenile yellowfin tuna (Thunnus albacares), and to provide references for the protection of marine ecosystems and wild stocks of tuna, we took juvenile yellowfin tuna Average body length of (18.21±1.09) cm and average body mass of (354.98±149.77) g as research subjects, and set the pH gradients of 8.1, 7.6, 7.1 and 6.6 to determine the malondialdehyde (MDA), superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), peroxidase (POD), acid phosphatase (ACP), alkaline phosphatase (AKP) and lysozyme (LZM) activities. The results show that after 48 h, the juveniles in pH 6.6 group showed mortality. Except for the control group, the activities of SOD, CAT, POD and LZM in gills were higher (P<0.05) than those of the other tissues, with the activities of SOD, CAT and POD peaking at pH 7.1, and the LZM activity peaking at pH 6.6; the GSH-Px activity in skin was higher than that in the other tissues (P<0.05), peaking at pH 7.1. In the liver, the MDA content was the most accumulated and the ACP activity was the highest (P<0.05), with the former peaking at pH 6.6 and the latter peaking at pH 8.1; the AKP activity in red muscle was significantly lower than that in the other tissues (P<0.05), reaching its lowest level at pH 7.1. The results show that there is some resistance of immune function and antioxidant defense system in juvenile yellowfin tuna when seawater pH is above 7.1, but has partial immune dysfunction when pH is lower than 7.1.