Based on air quality automatic monitoring data and meteorological data, this study analyzed the variation of PM2.5, PM10, SO₂, and NO₂ during two dust weather cases that occurred in Beijing on 28 February and 9 March 2013. The results showed that:(1) Concentrations of PM2.5 and PM10 showed clearly opposite change characteristics when the dust arrived at Beijing:The PM10 concentration increased by 50.8% and 202.4% in two hours, respectively, with a maximum of over 800 μg m^-3. On the contrary, the PM2.5 concentration decreased by 58.3% and 50.9%, respectively, both to lower than 35 μg m^-3. The dust weather had a significant effect on reducing the concentration of PM2.5 in Beijing. (2) Although concentrations of PM2.5 rose slowly in a few hours before the dust arrived, the concentration of PM2.5 declined consistently during the dust arrival period. The PM2.5/PM10 ration dropped from above 0.75 to below 0.25. In both cases, the day before the dust day, daily concentrations of PM2.5 were greater than 150 μg m^-3, and air quality was at a serious pollution level, implying that fine particulate matter contributed to the air pollution. After the arrival of the dust, air pollution was mainly caused by giant and large particulate matter.