Fog and haze can transform from/to each other under such a typical evolution pattern as “heavy haze (HH) – extremely dense fog (EDF) – heavy haze”. To explore the key meteorological factors for the transformation from HH to EDF, the near-surface circulation patterns of 247 EDF and 96 HH cases in the central area of the Huaihe River Basin (HRB) at 08:00 (high-intensity period) for 40 years were objectively classified respectively, using the multi-element oblique rotation principal component analysis method, together with the ERA5 reanalysis data. Based on the results of classification, the formation mechanisms for EDF under various circulation patterns were analyzed, and the differences of meteorological conditions in the surface and boundary layer of HH and EDF with similar large-scale circulation patterns were compared. The results show that: (1) The circulation of EDF can be divided into five types, with a cold high in northwest or northeast China for each type, and a sub-synoptic system in the HRB, such as weak high, frontal or inverted trough; The studied area is located in the north (33%) or inside (19%) of the weak high, in front of the front (29%) or the invert trough (11%) or at the bottom of the cold high (7%), prevailing with southwest, south or east wind with average wind speed lower than 1.6 m/s at ground level. At 925 hPa, the study area is located in the warm ridge and low humidity area, with an average wind speed of 2 m/s. In the vertical, the relative humidity (RH) decreases rapidly with height, and to below 60% at 925hPa or 850hPa, which is conducive to the surface radiation cooling, with an average temperature decrease range larger than 3℃, forming a deep near-surface inversion, with a temperature difference of 2-4 ℃ between 975 hPa and the ground. (2) The near-surface circulation for HH can be divided into three types, which are all related to high pressure system. The study area is located in the front (56%), bottom (19%) or rear (26%) of the surface high, but there is no sub-synoptic scale system or the system is weak in the HRB. (3) The reasons for HH cannot develop into EDF include: ① insufficient water vapor source (due to northerly wind on the ground); ② low cooling range at ground level (the average lower than 3.1 ℃) with relative high wind speed (the average over 2.2 m/s). (4)The sub-synoptic scale system near the study area is the key reason for whether HH can develop into EDF. This system determines local meteorological conditions, such as whether there are light winds, significant cooling, and sufficient sources of water vapor. The inter-monthly variation and generation and disappearance time of EDF under various circulation patterns are also analyzed.