Herein, a novel visible-light-responsive g-C3N4/diatomite/MnO2composite was successfully fabricated through a simple redox reaction method. The structure and morphology of the sample are mainly characterized by X-ray diffraction (XRD), photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and ultraviolet–visible spectroscopy (UV–Vis). In this paper, the photo-catalytic activity of the sample was evaluated by degradation RhB under visible-light irradiation. The results shown that, compared with g-C3N4/diatomite composite, MnO2, g-C3N4, diatomite, the prepared g-C3N4/diatomite/2.5%MnO2composite exhibits better photo-catalytic activity and stability. At the same time, the effect of different MnO2additions on the photo-catalytic activity of the composite material was further analyzed. The results indicated that the g-C3N4/MnO2/diatomite composites exhibit highest photo-catalytic activity when the adding amount of MnO2reached 2.5%. The degradation rate of the g-C3N4/diatomite/2.5%MnO2is 93% after recycling for three times, showing good stability and reusability. Moreover, the mechanism of catalytic performance enhancement also has been discussed.