The typical fog and haze episode in the Sichuan basin from December 16, 2014 to January 27, 2015 was selected according to the method of determination of fog and haze days at a single station and the regional fog and haze process. Based on air quality index (AQI), mass concentration of pollutant, meteorological element characteristics and atmospheric circulation, the characteristics and mechanism of the generation, evolution and transformation of this continuous fog and haze weather were studied. The results are as follows: (1) The fog and  haze process was characterized by strong intensity, long duration and wide range. (2) Both mass concentration of pollutant and AQI were highly consistent with the fog and haze weather process, and the main pollutant in this fog and haze process was PM2.5, followed by PM10. (3) It can be seen from the process of formation, evolution and transition of a longlasting severe fog and haze episode that there were stages of pollutant accumulation with different intensities, reaching the peak value and rapidly weakening. Therefore, the intensity of fog and haze weather process was closely related to weather situation and vertical structure of boundary layer. The average temperature was 1.24 ℃ higher than the normal years, the rainfall was 34.77% lower than the same period, relative humidity was 2.67% lower than the same period, sunshine hours was 10.33 hours more than the same period, and wind speed was close to or slight higher than that of the normal years. The stable atmospheric circulation feature provided favorable environmental field for fog and hazy weather and heavy air pollution. Strong temperature inversion layer, sinking motion in the boundary layer and convergence in weak wind fields on the ground prevented water and pollutants from diffusing to the upper air.