A composite photocatalyst (Ho/TiO2/Fe3O4) with Ho-doped anatase titanium dioxide (Ho/TiO2) shell and a magnetite core was prepared by coating photoactive Ho/TiO2 onto a magnetic Fe3O4 core through the hydrolysis of tetrabutyltitanate (Ti(OBu)4, TBT) in water/oil (w/o) microemulsion with precursors of Ho(NO3)3 and TBT in the presence of Fe3O4 nanoparticles. The morphological, structural and optical properties of the prepared samples were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV-Vis diffusive reflectance spectroscopy (UV-Vis DRS). The effect of Ho ion content on the photocatalytic activity was studied. The photodegradation behavior of the prepared photocatalyst under UV and visible light was investigated in aqueous solution using methyl orange (MO) as target pollutant. The results showed that the prepared photocatalyst was activated by visible light and used as effective catalyst in photo-oxidation reactions. In addition, the possibility of cyclic usage of the prepared photocatalyst was also confirmed. Moreover, Ho/TiO2 was tightly bound to Fe3O4 and could be easily recovered from the medium by a simple magnetic process. It can therefore be potentially applied for the treatment of water contaminated by organic pollutants.