Abstract: To investigate the formation mechanisms of severe dense fog (SDF), back-trajectory cluster analysis, together with composite analysis, was employed. Atmospheric circulation patterns were classified at different pressure levels, from the surface to 500 hPa, for 103 SDF and 48 heavy haze (HH) cases in northwestern Anhui from 2005 to 2019 at 0800 BJT (Beijing time). Vertical changes in circulation patterns, trajectory directions, and the boundary layer structure between SDF and HH were compared. The main results are as follows: (1) During SDF events, at 1000 hPa, the study area was generally located south of a weak high-pressure system, ahead of a shallow trough or weak ridge, with small variations in relative humidity across circulation types. At 925 and 850 hPa, the area was mainly located ahead of a trough or within a saddle pressure field, with relative humidity generally below 50% at 850 hPa. By contrast, during HH events, from the surface to 850 hPa, the study area was mainly located within regions of weak pressure gradient southeast or south of a cold high-pressure system, with large humidity variations among circulation types. Notably, SDF and HH events occurred within a cold trough at 1000 hPa, whereas both were located within a warm ridge at 925 and 850 hPa. At 500 hPa, the geopotential height difference between southern and northern Anhui remained consistently at around 80 gpm during SDF events; however, it was highly variable during HH events, ranging from 40 to 120 gpm. (2) During SDF events, near-surface trajectories exhibited dispersed origins. With increasing altitude, the proportion of northwesterly trajectories increased significantly, reaching 67% at 1500 m. In contrast, during HH events, near-surface trajectories were mainly from northerly directions (82%), whereas the proportion of southwesterly trajectories increased with height. (3) Based on trajectory clustering results at 1500 m, vertical profiles of temperature, humidity, and wind speed in each cluster during SDF were relatively consistent and exhibited similar characteristics. By contrast, these profiles were more dispersed during HH. In summary, the location of the study area within synoptic systems and the vertical variation of trajectory directions play important roles in the formation of SDF.