The work aims to ensure that the designed photocatalytic evaluation system can accurately characterize degradation effects of photocatalytic coating on 0.25 μL/L benzene hydrocarbon gas by verifying and designing advanced photocatalytic evaluation system. PID photo ionization detector was firstly used to monitor dynamic degradation rate of pollutants. Materials and technology were improved so that 0.25 μL/L level of benzene gas degradation could be detected by the system. A series of properties characterization test were performed to two TiO2 photocatalysts with benzene gas as pollutant. The evaluation system could effectively detect photocatalytic difference between the two photocatalysts as 11.7%～57.3% and 13.4%～67.4% respectively in 5～20 W/m2 visible light. The influence of flow rate and concentration of the pollutants on the degradation efficiency were investigated as well. It was found that pollutant concentration had minor effect on degradation efficiency provided with 0.05～0.4 μL/L of benzene gas. Otherwise, the degradation efficiency reduced from 43.6% to 25.9% and 52.9% to 29.9% respectively as flow rate of pollution gas reduced from 30 mL to 90 mL. The designed system is accurate and stable enough. Its high detection limit can characterize degradation effects of photocatalytic materials on trace gaseous organic pollutants, hence it can be widely used in scientific research of gas-solid two phase reaction and product evaluation in the filed of photocatalysis.