The concentration of PM₂.₅ decreased very rapidly from 18:00 to 23:00 on 17th Mar. 2015 in Beijing area. No cold air bringing strong north wind influenced Beijing. The reason leading to the clean away of PM₂.₅ was discussed. The results showed that a boundary layer jet played a key role. The ventilation in the boundary layer went up with the enhancement of southwesterly wind speed, which was favorable to the dilution of pollution. Besides, the development of jet also caused the increase of vertical wind shear. As a result, the turbulence in the boundary layer became more obvious and the mixing layer height rose. Furthermore, the geostrophic vorticity at the top of mixing layer was positive at 20:00 on 17th Mar. It means that the direction of Ekman-Pumping was upward. So, the pollution near the surface was brought to upper levels and transported downstream by the jet. The development of boundary layer jet attributed to inertial oscillation and atmospheric baroclinicity.