TiO2 can be used as a photocatalyst because of its semiconductor property. When TiO2 is doped with transition metal ions, its electronic properties may be modified. In this work, nanosized Sn4+-doped TiO2 (TiO2-Sn4+) particles have been synthesized by a sol-gel process at different temperature. Their microstructure and crystal structure depending on the calcination temperatures were investigated using XRD and SEM techniques. The photocatalytic degradation of diesel pollutants in seawater was investigated by using Sn4+-doped nanometer TiO2 under UV irradiation. In the experiment, the effect of different ratio of Sn4+ doped into TiO2, the effect of calcination temperature of Sn-doped nano-TiO2, the effect of Sn4+-doped nanometer TiO2 dosage, pH value and photocatalytic degradation reaction time, respectively, on the removal of diesel pollutants in seawater was investigated. On the basis of the results of these experimental data, the optimal experimental conditions for photocatalytic degradation of diesel are tried through the orthogonal test. The results reveal that when the ratio of Sn4+ doped into TiO2 was 3% wt, the calcination temperature of Sn-doped nano-TiO2 was 400°C, the Sn4+-doped nanometer TiO2 dosage was 1 g/L, pH value was 8 and photocatalytic degradation reaction time was 1.5h, respectively, the removal rate of diesel pollutants in seawater was expected to reach 88.00 %.