Abstract: Cu²⁺/SnO₂ composite nanophotocatalyst was successfully prepared by chemical precipitation method. The particle size and morphology of the composite nanophotocatalyst were characterized by XRD and SEM.The effects of Cu²⁺/SnO₂ composite nanophotocatalyst on the degradation of marine diesel pollutant were studied under different conditions of ultraviolet illumination, catalyst doping ratio, catalyst calcination temperature, catalyst input, initial diesel concentration and illumination time.The results indicate that self-made composite nanophotocatalyst can effectively degrade diesel pollutant in seawater. It was found that the Cu²⁺/SnO₂ composite nanophotocatalyst with a copper/tin doping ratio of 0.03 and calcination temperature at 400 °C had gained the highest removal rate 86.98% of diesel when 0.2 g/dm³ dosage of catalyst was added in the solution with an initial diesel concentration 0.15 g/dm³ and H₂O₂ concentration 0.2 g/dm³ at pH 7 and under ultraviolet light for 3 hours. The Cu²⁺/SnO₂ composite nanophotocatalyst can be practically applied to the ocean after loaded by polypropylene balls for the recycle.