Herein, a kind of Fe3O4@C@Ru hybrid nanocomposites was successfully synthesized by a three-step process. The as-prepared products were characterized by X-ray powder diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy, UV-visible diffuse reflectance spectrometry and photoluminescence emission spectrometry. The Ru nanoparticles loaded on the surface of carbon shell on Fe3O4@C nanospheres were nearly spherical with an average diameter of less than 5 nm. The photodegradation of methylene blue (MB) was carried out under the irradiation of simulated visible light to investigate the catalytic activity of the introduced nanomaterial. The results showed that the catalytic activity of Fe3O4@C@Ru nanocomposites in the MB photodegradation was higher than that of Fe3O4 nanospheres or of Fe3O4@C nanospheres, as they could degrade 92.70% of the dye in 140 min of simulated sunlight irradiation without any other oxidant or reductant. The photodegradation reaction was first-order with the rate constant of 0.0176 min(-1) at room temperature. The degradation rate of Fe3O4@C@Ru nanocomposites decreased very slightly after five cycles. Furthermore, they could be easily separated and recycled by using an external magnet. It indicated that the magnetically recoverable Fe3O4@C@Ru nanocomposites had a very broad application prospect in the field of environmental protection.