The present work reported the synthesis of visible-light-induced cobalt-doped zinc ferrite (Zn1-xCoxFe2O4) photocatalysts via a facile reduction-oxidation route, which involved rapid reduction of Fe3+ and Co2+ cations in colloid mill reactor, followed by oxidation of iron and cobalt nuclei and structural transformation under hydrothermal conditions. The structural and optical properties of materials were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV-vis diffuse reflectance spectra. The results indicated that metallic Co and Fe nuclei could be obtained by reduction in the colloid mill, and Zn1-xCoxFe2O4 nanocrystals with uniform size were successfully achieved. Furthermore, compared to ZnFe2O4, Zn1-xCoxFe2O4 samples (x = 0.03, 0.1, 0.2) exhibited enhanced photocatalytic activity in the degradation of methylene blue under visible light irradiation. Especially, the Zn0.8Co0.2Fe2O4 sample had the highest photocatalytic activity, which was mainly attributable to its smaller band edge. Since as-synthesized Zn-Co ferrites nanocrystals have the advantages of intrinsic chemical stability and magnetic property, it can be expected that they may have potential application in the field of industrial photodegradation of organic pollutants.