The continuative fog event lasted from 29 November to 4 December 2004 is analyzed. The results indicate that the northwestern airflow and the warm and humid airflow from southwest at upper levels are important factors of the continuative fog event; the surface relative humidity was up to 94% and North China was controlled by the stable Continental High and a saddleshaped field, so there was abundant water vapor, stable stratification, and very small wind speed at lower levels, leading to the fact that water vapor could not diffuse timely; the upperlevel sinking air and lowerlevel rising air led jointly to the stable stratification, so heavy fogs occurred.