Abstract: Most of the previous studies on the heavy pollution process in winter in Beijing focused on aerosol and paid less attention to fog in the process. Based on the L-band radiosonde and surface meteorological observation data of Beijing southern suburb observatory, as well as satellite data, such as MODIS and VISSR, the fogs during the period of heavy pollution in Beijing from January 12 to 15, 2013 were analyzed. The results showed that two radiation fog processes and two advection fog processes occurred in four days during the period. Each day's fog was completely different in terms of type, vertical distribution, formation process, stratification and so on. A mature radiation fog occurred on the morning of January 12, and the generation and development of radiation fog made the stratification of surface layer change from stable condition to near-neutral. Advection fog with 2 connected layers each with different origin occurred on the morning of January 13. The arrival of advection fog dissipated the fog in the surface layer and increased the vertical diffusion capability in the low atmosphere. Another radiation fog occurred on the morning of January 14, and the development was limited to its incipient stage and the lifetime was just about 2 h. There was ground inversion when the fog occurred. Deepened advection fog occurred on the morning of January 15. The height of fog top raised about 500 m compared with the last night fog. Given the complexity and variability in the fog formation, its vertical structure and influence on stratification during heavy pollution in winter in Beijing, research on fog layer and its effects on vertical diffusion and radiation during the heavy pollution needed to be strengthened.