In this paper, the effectiveness of the composite photocatalyst was studied by using manganese dioxide (MnO2)/zirconium dioxide (ZrO2) to degrade diesel pollutants in seawater under visible light.The MnO2/ZrO2 photocatalyst was prepared by co-precipitation and characterized by scanning electron microscopy, X-ray powder diffraction, energy-dispersive spectroscopy and UV-Vis diffuse reflectance spectroscopy analysis. This is the first report on a comprehensive analytical study on the effect of various physio-chemical parameters on diesel degradation using the synthesized MnO2/ZrO2 photocatalysts. The effects of doping ratio of MnO2/ZrO2, dosage, initial diesel concentration, calcination temperature, concentration of H2O2 solutions and illumination time on the diesel degradation were investigated. The degradation of diesel pollution in seawater was optimized by orthogonal experiment. According to the results, the prepared samples were monoclinic form and the MnO2 was successfully doped into the bulk ZrO2. The absorption edge of the MnO2/ZrO2 photocatalysts exhibited red shift, and this red shifts imply enhanced photon absorption under visible light compared with the pure ZrO2. The results showed that under optimum reaction conditions, the degradation rate can reach 92.92%. The result of this study will enable ZrO2 to make more effective use of sunlight and improve the actual value of photocatalytic technology in the field of contaminant treatment.