Introducing oxygen vacancies into zirconia is an effective strategy to enhance its light absorption and photocatalytic performance. However, the cost-efficient preparation of oxygen-deficient zirconia (ZrO_2-x) remains challenging, which severely limits its broad application. In this study, flash sintering treatment was presented to fabricate ZrO_2-x bulk in a very short time of 90 s. It was found that oxygen vacancies were introduced into ZrO₂ bulk through electrochemical reduction reactions. The as-prepared ZrO_2-x with black color exhibited the excellent optical absorption capability, small band gap (2.09 eV for direct and 1.67 eV for indirect), and reduced conduction band energy, which is ascribed to the generation of oxygen vacancies and reduction of Zr cations. The as-prepared ZrO_2-x showed the remarkable photocatalytic activity due to the excellent solar light absorption and low recombination rate of electron-hole pairs. Flash sintering treatment provides a cost-efficient approach for rapidly fabricating ZrO_2-x bulks with high concentration of oxygen vacancies, which can also be applied to other materials.