Heteroatom doping and defect engineering have been proposed as effective ways to modulate the energy band structure and improve the photocatalytic activity of g-C₃N₄. In this work, ultrathin defective g-C₃N₄ was successfully prepared using cold plasma. Plasma exfoliation reduces the thickness of g-C₃N₄ from 10 nm to 3 nm, while simultaneously introducing a large number of nitrogen defects and oxygen atoms into g-C₃N₄. The amount of doped O was regulated by varying the time and power of the plasma treatment. Due to N vacancies, O atoms formed strong bonds with C atoms, resulting in O doping in g-C₃N₄. The mechanism of plasma treatment involves oxygen etching and gas expansion. Photocatalytic experiments demonstrated that appropriate amount of O doping improved the photocatalytic degradation of rhodamine B compared with pure g-C₃N₄. The introduction of O optimized the energy band structure and photoelectric properties of g-C₃N₄. Active species trapping experiments revealed ·O₂^– as the main active species during the degradation.