This paper presents a novel visible-light-driven catalyst, a SO42-/MoOx/MgF2 composite, which was synthesized by a simple solution method. Multiple techniques, including Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance spectroscopy (DRS) were applied to investigate the physical and photophysical properties of the catalysts. The photocatalytic activities were evaluated in the degradation of acetone in gas phase. In the photodegradation of acetone, the highest conversion was obtained over a catalyst containing 5 mol % molybdenum. The XRD and Raman characterizations indicate that the molybdenum oxide was highly dispersed in the MgF2 matrix. These MoOx species might be the active sites of the catalysts, which is the reason for the visible-light response of the composite catalyst. The MgF2 matrix acts to isolate the MoOx species and retard the electron hole pair recombination. When the molybdenum concentration is >5 mol %, crystalline MoO3 phase was observed. The large MoO3 particle would decrease the separation efficiency. Thus, the photocatalytic activity was reduced. Besides the molybdenum concentration, the calcination temperature also shows a great effect on the activity. A sulfated 5 mol % MoOx/MgF2 catalyst that was calcined at 350 degrees C showed the highest photocatalytic activity. Based on the results of the characaterization, the origin of the high activity was discussed. The light absorption ability and the MoOx size effect are considered as the key factors.