This study aims to construct an MFC with a photosynthetic algae cathode, which is maintained by self‐capturing CO2 released from the anode and utilizing solar energy as energy input. With this system, a maximum power density of 187 mW/m2 is generated when the anode off gas is piped into the catholyte under light illumination, which is higher than that of 21 mW/m2 in the dark, demonstrating the vital contribution of the algal photosynthesis. However, an unexpected maximum power density of 146 mW/m2 is achieved when the anode off gas is not piped into the catholyte. Measurements of cathodic microenvironments reveal that algal photosynthesis still takes place for oxygen production under this condition, suggesting the occurrence of CO2 crossover from anode to cathode through the Nafion membrane. The results of this study provide further understanding of the algae‐based microbial carbon capture cell (MCC) and are helpful in improving MCC performance.