Using H2SO4 and KMnO4 as the oxidant, the ultrathin oxygen-doped g-C3N4 nanosheets was prepared through ultrasonic assisted oxidative exfoliation of original graphite phase carbon nitride (g-C3N4) synthesized by thermal polymerization. Based on the characterization of HRTEM, XRD, AFM and XPS, the effect of ultrasound assisted oxidative exfoliation on the morphology and structure of pristine g-C3N4 was investigated; the photocatalytic performance of ultrathin oxygen-doped g-C3N4 nanosheets was analyzed by photocatalytic degradation of Congo red under visible light; and the enhancement mechanism of photocatalytic performance of ultrathin oxygen-doped g-C3N4 nanosheets was also explored by means of UV-vis-DRS, EIS and PL analysis. The results showed that, the ultrathin oxygen-doped g-C3N4 nanosheets with a specific surface area of 58.45 cm2 g-1 and thickness of 1.08 nm was obtained after 6 h of ultrasound assisted oxidative exfoliation of pristine g-C3N4; The ultrathin oxygen-doped g-C3N4 nanosheets（200 mg L-1）degraded Congo red (20 mg L-1) by 83% within 120 min, showing a good photocatalytic performance; Compared with pristine g-C3N4, the ultrathin oxygen-doped g-C3N4 nanosheets is more conducive to the exposure of catalytic active site, but also the separation and transportation of photogenerated carriers, which has better photocatalytic performance.