Green tide is a high biomass ecological disaster caused by the proliferation of green algae in intertidal zone. Its outbreak is not only driven by environmental factors such as temperature and nutrients, but also closely related to its own photosynthetic capacity. In this study, Ulva intestinalis and Ulva expansa, the two green tide species, were selected for the outdoor culture experiment. The photosynthetic pathway, antioxidant physiological characteristics and their corresponding relationship with photosynthetic products of the two species were detected and compared under the conditions of high temperature and high light intensity in summer. The results showed that there were significant differences in photosynthetic pathway and antioxidant capacity between U. intestinalis and U. expansa. The key enzyme activity of C₄ photosynthetic pathway was highly expressed in the process of photosynthesis of the former, which was significantly correlated with light and temperature. The key enzyme activity of C₃ photosynthetic pathway of U. intestinalis was inhibited by strong light at noon, and the change range of δ¹³C in tissue ranged from −17.1‰ to −15.7‰, which indicated that C₃ and C₄ pathway might be involved in its photosynthetic cooperation. For U. expansa, the expression of key enzymes in C₄ photosynthetic pathway was weak and there was no significant correlation with light and temperature. Also, there was no obvious photoinhibition in C₃ photosynthetic pathway and the range of δ¹³C in tissue is from −23.5‰ to −21.9‰, indicating that the photosynthesis mainly depends on C₃ pathway. In addition, U. intestinalis showed a strong antioxidant capacity in the process of culture, which may be closely related to its C₄ photosynthetic pathway under high temperature and high light conditions. The comparative study between U. intestinalis and U. expansa showed that there were significant differences in the initiation of C₄ photosynthetic pathway between algal species, and the comparative study on the photosynthetic mechanism of different green algal species needs further exploration.