Using SO₃ generating device and simulated flue gas wet desulfurization system, the effects of different sampling parameters such as flue gas reheating temperature, constant temperature water bath temperature, sampling rate, spiral tube size and number of absorption bottles on SO₃ acid mist collection efficiency were studied. The results showed that when the flue gas reheating temperature was lower than the acid dew point of flue gas, the SO₃ acid mist collection rate would decrease. When using the controlled condensation method, a large flow rate (10L/min) should be used, and a spiral condenser should be selected with a length of 400mm, which should be placed in a constant temperature water bath at a temperature of about 65 ℃. When sampling with alkaline absorption method, a small flow rate (1L/min) should be used, and three absorption bottles should be used in series. The temperature and humidity of the flue gas, the concentration of SO₂, and the coexisting components in the flue gas had great influence on the alkaline absorption method, but had little effect on the controlled condensation method. Therefore, controlled condensation method is more accurate when monitoring SO₃ acid mist in highhumidity flue gas after wet desulfurization.