In this paper, WRF (V3.9) is used to simulate the mountain-plain breeze circulation in eastern Chengdu (Longquan mountain) during a heavy pollution event on 7 December, 2016. The structure and evolution of the mountain-plain breeze circulation are discussed. Besides, the influence of the Aerosol Optical Depth (AOD) on the mountain-plain breeze circulation is examined by conducting a sensitivity experiment in which the AOD value is reduced. The Longquan mountain is fairly long from north to south and relatively narrow from east to west with low elevation. Results show that strong ground inversion existed in the winter during the heavy pollution event. Cross-mountain downslope wind circulation occurred in the night and late morning due to the imbalance of radiative heating between western and eastern slope and the dynamic forcing of background wind. The plain breeze began after the noon and ended at around 1700 LST. The maximum horizontal stretch of the plain breeze circulation could reach 3 to 4 times of the mountain width. Cross-mountain downslope wind circulation and mountain-plain breeze circulation varied greatly in the north-south direction. The former was stronger in the southern part of the mountain whereas the latter in the central and southern part was easily covered by vigorous turbulence. In the northern part, on the contrary, there existed significant plain breeze circulation. After reducing the AOD in the sensitivity experiment, the solar radiative heating and planetary boundary layer height both increased to a certain extent. Plain breeze circulation in the central and southern part became more ambiguous due to amplified turbulences. In the northern part, plain breeze circulation enhanced and lasted longer due to the elevated planetary boundary layer and the strengthened difference in sensible heat flux between the mountain and plain areas.