The pollutant trajectory, atmospheric circulation background, and meteorological factors of a heavy air pollution fog/haze event in Qingdao from 15 January to 22 January 2018, were analyzed using routine weather observation, air quality monitoring data, and ECMWF reanalysis data. The results indicate that during the early stages of pollution in Qingdao, the primary sources were the convergence and accumulation of dry and cold air from Mongolia and wet air from the Yangtze–Huaihe River basin. In the late stage, the major sources were the local air pollutants in Shandong Province, with PM10 being the primary pollutant. During the pollution process, controlled by the ural blocking high and cold low pressure in central Siberia, the East Asian transverse trough remained stable over the mid-high latitude area at 500 hPa. Over Qingdao, westerly airflow was dominant, with low wind speeds near the ground. As the transverse trough turned vertical, the zonal circulation changed to meridional, the cold air moved southward, the wind speed increased drastically, and the fog/haze dissipated rapidly. Under stable atmospheric circulation, when the surface layer had a weak wind or continuous land breeze, the ascending and descending motion of the air in the lower troposphere was weak, and the water vapor condition was favorable for the maintenance of fog/haze. Thus, by analyzing the relationship among relative humidity, PM2.5 concentration, and visibility at various stages of the process, it was observed that during the haze stage, the influence of both factors was comparable; moreover, during the fog stage, visibility was primarily affected by relative humidity, and accumulation and increase of PM2.5 concentration under static and steady conditions were the primary factors affecting visibility during the fog and haze mixing stage.