Abstract: Based on comprehensive observations of haze in North China during 2011, the genesis and microphysical structure characteristics of a dense fog process are analyzed. Results show that the fog process is under the control of high-pressure uniform pressure field when the wind speed on the ground is small. The inversion layer near the ground, sufficient water vapor and radiation cooling are important causes for the fog process. After the occurrence of fog, the ground temperature decreased obviously, and the inversion layer also increased, accompanied by the rise of the high relative humidity area, and the thickness of the fog body increased continuously. Compared with the radiation fog process in Nanjing during winter, the concentration of small particle number is larger and the content of liquid water is lower. This is because of the high aerosol concentration in North China and the weak water vapor transport. The average aerosol number concentration in this fog process is three times of that in Nanjing. The concentration of cloud condensation nuclei is positively related to the aerosol number concentration. With the decrease of temperature, aerosol particles continue to aggregate and grow, and become droplets. The increasing number of droplets competes for water vapor, which makes it difficult big particles for to form in the fog. The long-wave radiation cooling effect at night results in the formation of near-ground fog, which in turn enhances the cooling effect of long wave of the fog rapidly, and promotes the formation and collision of a large number of small droplets, providing positive feedback. The latent heat released by the formation of droplets promotes the lifting of the fog body and the enhancement of downward long-wave radiation, and makes the cooling on the ground weakened, which is a negative feedback. This fog process shows characteristics of burst enhancement. Within 10 minutes, the number and density of droplets increased significantly, the water content increased by three orders of magnitude, the droplet spectrum widened from 15 μm to 35 μm, and the visibility plummeted from 500 m to 70 m. The explosive growth of fog is due to the increase of long-wave radiation on the top of fog, which makes the temperature drop continuously and the supersaturation increase. The water vapor condensation and droplet condensation lead to the rapid growth of droplets and a double peak spectrum distribution. The emergence of large drop number concentration further accelerates the collision process and further widens the spectrum width.