Photosynthetic bacteria (PSB) wastewater treatment is a novel method that can simultaneously achieve wastewater purification and recourse recovery. Oxygen conditions can significantly influence PSB metabolism and wastewater treatment; however, the roles of key enzymes in this process have been unclear. This study investigated the effects of different oxygen conditions on respiratory enzymes (hexokinase, HK; phosphofructokinase, PFK; and pyruvate kinase, PK) and the photosynthetic enzyme RuBisCO of PSB. The results showed that HK activity was almost the highest under anaerobic condition; PFK activity was the highest (311 nmol/min.g) and PK the most quickly reached its peak under aerobic condition; and RuBisCO activity was the highest (107 nmol/min g) under anaerobic condition. The above results illustrated that HK might simultaneously regulate PSB growth and COD degradation, and its main role might be PSB growth regulation. PFK and PK might function in COD degradation while RuBisCO might function in PSB growth. Further, oxygen was beneficial for COD degradation and detrimental for PSB growth. This study provides a theoretical basis for the development of PSB wastewater treatment.