Low loading catalysts Ru/γ-Al₂O₃ and Ru–Ce/γ-Al₂O₃ (Ru: 0.3 wt%) were prepared by thermolysis of Ru₃(CO)₁₂, which exhibited high activity in Catalytic Wet Air Oxidation (CWAO) of high concentration organic pollutants by upflow packed-bed reactor with initial COD 200000 mg L⁻¹. XPS results revealed that two new Ru species (Ru_A and Ru_B) were detected due to the chemical interaction between Ru₃(CO)₁₂ and the active OH groups on the surface of Al₂O₃ during the process of Ru₃(CO)₁₂ incomplete decomposition, and the reduction of them can lead to more dispersed metallic phases. The TPR spectrum reveals that the catalyst with addition of Ce produced a strong interaction between Ru and the CeO₂–Al₂O₃ support, which was helpful for the catalytic wet air oxidation. Low loading catalyst Ru–Ce/γ-Al₂O₃ reduced in H₂ had the highest activity and the COD removal reached 99.5% above 245 °C. The operating parameters investigated included temperature, reactor pressure, gas flow rate and liquid hourly space velocity (LHSV). The results showed that the COD removal is considerably affected by the temperature; when temperature increased around 2–3 °C, COD removal was improved about 40% due to the obvious exothermic reaction. The experimental results indicated that the 0.3 wt% Ru–Ce/γ-Al₂O₃ catalyst has excellent activity and stability in the CWAO of high concentration organic pollutants in as packed-bed reactor during 100 hours.