Cerium dioxide (CeO₂) photocatalysts are used in treating environmental pollution and addressing the energy crisis due to their excellent oxygen storage capacities and abundant oxygen vacancies. In this paper, CeO₂ precursors were synthesized with different water-alcohol ratios via a solvothermal method, and CeO₂ photocatalysts with different Ce³⁺/Ce⁴⁺ ratios were obtained by changing the precursor calcination atmospheres (air, Ar) as well as the calcination time. The effects of CeO₂ with different Ce³⁺/Ce⁴⁺ ratios in photocatalytic degradations of methylene blue under visible light were investigated. X-ray photoelectron spectroscopy results showed that the surfaces of the samples calcined under Ar had higher Ce³⁺/Ce⁴⁺ ratios and oxygen vacancy concentrations, which reduced the band gaps of the catalysts and improved their utilization of visible light. In addition, the many Ce³⁺/Ce⁴⁺ redox centers and oxygen vacancies on the sample surfaces improved the separation and transfer efficiencies of the photogenerated carriers. The sample C2-Ar calcined under Ar showed a high adsorption capacity and excellent photocatalytic activity by removing 96% of the methylene blue within 120 min, which was more than twice the degradation rate of the sample (C2-air) prepared via calcination under air. Trapping experiments showed that photogenerated holes played a key role in the photocatalytic process. In addition, a synergistic photocatalytic mechanism for the Ce³⁺/Ce⁴⁺ redox centers and oxygen vacancies was elucidated in detail, and the sensitization of cerium dioxide by dyes aided the degradation of methylene blue.