Photosynthetic bacteria (PSB) wastewater treatment has the advantage of biomass recovery in together with pollutant removal. The effects of different Fe2+ concentrations on the biomass accumulation through regulating energy metabolism were investigated in PSB wastewater treatment. Results showed that the optimal Fe2+ dosage was 20 mg/L. Optimal Fe2+ content could significantly increase the biomass production (4800.9 mg/L) and COD removal (93.4%). Addition of 10–30 mg/L Fe2+ could shorten the hydraulic retention time of wastewater. Mechanism analyses revealed that different Fe2+ concentrations had different impacting mechanisms on biomass accumulation. Fe2+ constituted the dehydrogenase active center, and therefore proper addition of Fe2+ could improve energy production by up-regulating dehydrogenase activity, which was beneficial for biomass accumulation. With 20 mg/L Fe2+, the dehydrogenase activity and ATP production of PSB were improved by 48.1% and 42.4%, respectively. However, excessive addition of Fe2+ was harmful for biomass accumulation since the ions inhibited the dehydrogenase activity.