Sillenite-type photocatalytic material Bi₁₂MnO₂₀ with photo-absorptive ability in the spectrum range of 200-2200 nm was prepared by wet ball milling followed by calcination. The crystal structure of Bi₁₂MnO₂₀ can be viewed as the metastable cubic γ-Bi₂O₃ with the incorporation of Mn⁴⁺ into tetrahedral sites. The entry of Mn⁴⁺ into γ-Bi₂O₃ crystal creates a sub-bandgap of 1.25 eV appearing in the energy band structure of Bi₁₂MnO₂₀. The Bi₁₂MnO₂₀ photocatalyst can accelerate the transformation of O₂ in solution into superoxide radicals (·O₂^-), thereby improving charge separation and transfer during photocatalytic degradation. As a result, the degradation rate of over 95% to the malachite green (MG) with a concentration of 10 mg/L in the pH=8 solution by Bi₁₂MnO₂₀ is achieved under simulated sunlight. After four cycles of photo-degradation, the removal rate of MG by Bi₁₂MnO₂₀ remains at 74.9%.