In this study, a liquid-phase oxygen electrode system and an antioxidant enzymes kit were used to determine the photosynthetic characteristics and antioxidant system of Sargassum thunbergii at various pH and salinities. The net photosynthetic rates (P_n), pigment content, maleic dialdehyde (MDA) content and antioxidant enzymes activity of S. thunbergii were investigated. The major results included: (1) P_n rapidly decreased at pH 6.0 and pH 9.5 for 24 h, which were remarkably significant (P<0.05) compared with that of the control (pH 8.5) for 48 h. (2) Both superoxide dismutase (SOD) and catalase (CAT) activity increased significantly (P<0.05) at pH 9.5 for 48 h. And MDA content firstly increased, then decreased at pH 9.5 for 48 h. Compared with that of control (salinity 31), P_n decreased significantly (P<0.05) at salinity 15 for 24 h and at salinity 25 or salinity 20 for 48 h. However, P_n had some rise at salinity 25, 20 and 15 for 72h. (4) MDA content increased significantly (P<0.05) at first 24 h and then rapidly decreased at salinity 20. And both SOD and CAT activity increased significantly (P<0.05) at salinity 20 from 24 h to 48 h. MDA content increased significantly (P<0.05) with SOD, CAT activity relatively low at salinity 15 after 72 h. Above results indicated that photosynthetic capacity of S. thunbergii was inhibited at pH 6.0 and pH 9.5. S. thunbergii was damaged by reactive oxygen species (ROS) under strong alkaline stress. Antioxidant enzymes eliminated ROS readily at salinity over 15, but had low activity at salinity 15. In a word, these results revealed the change of photosynthetic capacity and provided reference for the regulating mechanism of antioxidant system response under adversity stress.