Models-3/CMAQ modeling system was used to simulate the spatial and temporal distribution of PM_2.5 pollution over Beijing during APEC, 2014(i.e. November 3 to 11, 2014). IPR, a process analysis tool embedded in CMAQ, was employed to quantify the contributions of different atmospheric processes to the PM_2.5 formation at two typical sites (i.e. Guanyuan and Dingling) during two short-time pollution processes (i.e. Nov. 4 13:00 to Nov. 5 12:00 and Nov. 10 13:00 to Nov. 11 12:00). The results showed that CMAQ reproduced the temporal variation and magnitude of PM_2.5 reasonably. Adverse synoptic system occurred on Nov. 4 and Nov. 10, resulting in two peak values of PM_2.5 (188 μg/m³ and 124 μg/m³). Elevated PM_2.5 levels didn't last long because of the pollution control measures and the cold anticyclone. During Nov. 4 13:00 to Nov. 5 12:00, horizontal transport was the primary contributor to the PM_2.5 at both Guanyuan and Dingling, with a contribution rate of 49.6% and 90.9%, respectively, indicating that Beijing was mainly affected by pollution transported from southern areas. During Nov. 10 13:00 to Nov. 11 12:00, PM_2.5 at Guanyuan site mainly came from local emission (78.8%), while PM_2.5 at Dingling site mainly came from relatively weak horizontal transport, demonstrating a local pollution characteristic. Vertical transport played a dominative role in the decrease of PM_2.5 in both pollution processes.