Porphyra yezoensis, an intertidal seaweed, experience a series of environmental changes, such as increasing CO2 concentration and solar UVR in the context of climate change. In the present study, the porphyra yezoensis collected form Gaogong island (34°N, 119°E) were used to investigate the response of photosynthetic electron transport performances to CO2-induced seawater acidification and UVR. Among all treatments, the significant decrease of Fv/Fm indicated a significant PSII photoinhibition in midday. YI showed no significant difference in the diurnal cycle, indicating that PSI kept active. The high activity of PSI might maintain the functional integrity of the photosynthetic apparatus in exposing to high light or UV-A. However, both PSII and PSI suffered severely photoinhibition under UV-B treatments. When exposed to UVR, the significant decrease of Wk implied that the donor side of PSII was inhibited, especially in UV-B treatments. The significant decrease of φo indicated an inhibition in the PSII acceptor side under low pCO2, and the decrease of φRo indicated an inhibition in the PSI acceptor side. In conclusion, UV showed significant negative effect on the photosynthetic performance of porphyra yezoensis, particularly in UV-B; PSII showed a significant decrease in midday, while PSI kept active throughout day, the active PSI may play an important role in balancing the redox state of photosynthetic electron chain in porphyra yezoensis; compared with low pCO2 treatment, high pCO2 could alleviate the reduction of PSII acceptor side.