Abstract: To investigate the formation mechanisms of severe dense fog (SDF), a back-trajectory-cluster analysis, together with composite analysis, was employed to objectively classify atmospheric circulation patterns at different levels from sea level to 500 hPa for 103 SDF cases and 48 heavy haze (HH) cases that occurred at 08:00 (Beijing time) in northwestern Anhui from 2005 to 2019. The vertical changes in circulation patterns and trajectory directions, as well as the boundary layer structure, between SDF and HH were compared. The main results are as follows: ①During SDF events, at the 1000hPa, the study area was generally located south of a weak high-pressure system, ahead of a shallow trough or weak ridge, with small variation in relative humidity among all circulation types. At the 925 hPa and 850 hPa, the area was mainly located ahead of a trough or within a saddle field, with relative humidity generally below 50% at 850hPa. In contrast, during HH events, from the surface to 850 hPa, the study area lay mainly within uniform-pressure zones southeast or south of a cold high-pressure system, with large humidity variations among all circulation types. Notably, both weathers occurred within a cold trough at 1000 hPa, whereas they were located within a warm ridge at 925hPa and 850hPa. At the 500hPa, the geopotential height difference between southern and northern Anhui remained consistently around 80 gpm during SDF events, but highly variable during HH events, ranging from 40 to 120 gpm. ②During SDF events, near-surface trajectories exhibited dispersed origins. With increasing altitude, the proportion of northwesterly trajectories rose significantly, reaching 67% at 1500m. In contrast, during HH events, near-surface trajectories were mainly from northerly directions (82%), while the proportion of southwesterly trajectories increased with height. ③Based on the trajectory clustering results at 1500 m, vertical profiles of temperature, humidity and wind speed of each group during SDF were relatively concentrated and exhibited similar characteristics. In contrast, these profiles were more dispersed during HH. In summary, the location of the study area within synoptic system and the vertical variation of the trajectory directions play an important role in the formation of SDF.