Abstract: Using CO₂ as a renewable carbon source for the production of high-value-added fuels and chemicals has recently received global attention. The photoelectrocatalytic (PEC) CO₂ reduction reaction (CO₂RR) is one of the most realistic and attractive ways of achieving this, and can be realized effectively under sunlight illumination at a low overpotential. Oxygen-incorporated carbon nitride porous nanosheets (CNs) were synthesized from urea or melamine by annealing in nitrogen or N₂/O₂ gas mixtures. They were used as the photoanode with Bi₂CuO₄ as the photocathode to realize PEC CO₂ reduction to the formate. The electrical conductivity and the photoelectric response of the CNs were modified by changing the oxygen source. Oxygen in CNs obtained from an oxygen-containing precursor improved the conductivity because of its greater electronegativity, whereas oxygen in CNs obtained from the calcination atmosphere had a lower photoelectric response due to a down shift of the energy band structure. The CN prepared by annealing urea, which served as the source of oxygen and nitrogen, at 550 °C for 2 h in nitrogen is the best. It has a photocurrent density of 587 μA cm⁻² and an activity of PEC CO₂ reduction to the formate of 273.56 µmol cm⁻² h⁻¹, which is nearly 19 times higher than a conventional sample. The CN sample shows excellent stability with the photocurrent remaining constant for 24 h. This work provides a new way to achieve efficient catalysts for PEC CO₂ reduction to the formate, which may be expanded to different PEC reactions using different cathode catalysts.