A comprehensive field experiment of fog was carried out in the north suburb of Nanjing in December 2015. The burst reinforcement process of heavy fog after the rain was observed twice on 20-21 December. Using the conventional meteorological data, boundary layer profiles, fog droplet spectrum and so on, the weather background and boundary layer structure were analyzed, and some reasons about burst reinforcement of fog were discussed. Results show that the high humidity environment at ground and surface layer after rain provides sufficient moisture for the formation of fog, and the stable weather stratification controlled by the winter cold high-pressure in Nanjing and the radiation cooling at night are highly favorable to the generation of radiation fog. The burst reinforcement of fog is mainly related to the cooling and wetting. The strong cooling caused by the enhancement of upward long wave radiation at ground at the sunny night, and the increased moisture at near surface caused by the strong evaporation at ground after sunrise can directly give rise to the burst reinforcement of fog. The formation of the strong ground inversion layer is the key to the burst reinforcement of fog, and is favorable to the accumulation of moisture near the ground. The generation of the ultra-low-level jet will speed up the reinforcement of the ground inversion layer.