Abstract: SO₃ is one of the most important precursors of condensable particulate matter, and easily transferred into blue flue gas after emission, which causes adverse effects on the atmospheric environment. SO₃ is also corrosive and hazardous gas. It is necessary to measure and monitor its concentration in flue gas. At present, the results and accuracy for methods monitoring SO₃ in flue gas are difficult to judge and equipments need to be calibrated regularly. However, SO₃ is reactive and difficult to store in the tank and must be generated on-site. The lack of ready-made standard gas is the major limiting factor for accurate measurement and monitoring of SO₃. In this paper, a SO₃ generator was developed based on the principle of catalytic oxidation. Its performance, stability, and influencing factors were evaluated. The conversion ratio was stable at 98.5% on the condition that injecting 1000 ppm(μmol/mol) SO₂ at a flow rate of 0.1 L/min, loading 600 mg vanadium catalysts (40~60 mesi), and setting the conversion temperature at 430 ℃. By diluting the highly-concentrated SO₃ with clean air of different flow rates, the generator linearly outputs SO₃ standard gas in the range of 10 ppm to 1000 ppm, which can be used for calibration of measurement and monitoring of SO₃.