Photocatalytic water splitting is an environmentally friendly technique for oxygen evolution. Vacancy-rich layered materials with good electromigration properties are of great interests. Herein, we synthesized fullspectrum responsive vacancy-rich BiO2-x ultra-thin nanosheets (UTNSs). The band gap is narrowed and the edge of valence band is increased in the BiO(2-x )UTNSs, which are responsible for the enhanced light response and are confirmed by UV-Vis-NIR diffuse reflectance spectra (DRS), electronic spin resonance (ESR), X-ray photoelectron spectroscopy (XPS) and valence measurement. Compared to BiO2-x nanosheets (NSs), BiO2-x UTNSs exhibits enhanced photocatalytic performance for oxygen evolution under full-spectrum irradiation, which can be attributed to the oxygen vacancies. The presence of oxygen vacancies in BiO2-x UTNSs promote the oxidation ability. This finding provides defect level understanding for developing highly efficient full-spectrum light responsive photocatalysts.