To provide a basis for local fog and haze forecast in Taizhou, a heavy fog-haze episode which occurred from January 12th to 15th in 2020 is analysed based on conventional meteorological observation data, NCEP reanalysis data (1°×1°) and air quality data of environmental monitoring station from the aspects of evolution features, mechanism and backward trajectory. The results show that: Both the fog and had have diurnal variation. The PM_2.5 and PM₁₀ concentrations AQI were slightly higher than those of the fog during the process, related to the wet settlement caused during the fog. The cold air from the east road negatively impacted Taizhou, making air pollutants gather locally. More than 90% relative humidity, within 1 m·s^-1 near-ground wind speed, and weak vertical exchange were significant conditions for fog formation, from night to early morning. During the day, with the relative humidity weakened to 80%, the near-ground airfield increased to 2 m·s^-1, and higher air pollutants concentrate, the fog developed to haze. On the 13th, the warm advection increased above 900 hPa, causing a temperature inversion in the boundary layer. At the same time, relative humidity was over 90%, which strengthened the fog and haze. Finally, by analysing the backward trajectory, it was found that the air mass below 500 m caused the hazing process to be stable. On the 14th, the clean air mass above 500 m was supplied to lower levels, conducive to the diffusion of haze. On the evening of the 15th, the high wind and precipitationed ended the fog-haze process.