A new magnetically separable visible-light photocatalyst, magnetite/Bi-doped carboxylate-rich carbon spheres (Bi-MCRCSs), was synthesized under ultrasonic irradiation by using magnetite/carboxylate-rich carbon spheres (MCRCSs) as a precursor. The Bi-MCRCSs showed much better photocatalytic activity than MCRCSs in the degradation of methylene blue (MB) under visible-light irradiation (λ > 420 nm). Compared with MCRCSs, the Bi-MCRCSs show more intensive photoabsorption in the whole UV and visible region. In particular, Bi-MCRCSs display a broad absorption band centered at 550 nm, which give Bi-MCRCSs advantages over MCRCSs in the utilization of visible light for the degradation of organic pollutants. Owing to the Bi3+ doping, the BiIII/BiIV redox cycle can be established in the FeII/FeIII redox cycle system, which result in the establishment of dimetallic FeII/FeIII and BiIII/BiIV photoredox cycles for Bi-MCRCSs. This dimetallic FeII/FeIII and BiIII/BiIV photoredox cycle possesses a significantly enhanced photocatalytic degradation rate compared to that of the monometallic photocycles of FeII/FeIII for MCRCS. The synergistic effects between the FeII/FeIII cycle and the BiIII/BiIV cycle promote the regeneration of FeII ions in FeII/FeIII cycle and, hence, accelerate the degradation of organic pollutants. In addition, the magnetic saturation (Ms) value is about 20 emu/g. After completion of the reaction, the Bi-MCRCSs could be rapidly separated under an applied magnetic field.