In order to study the physical-chemical processes of fog in Nanjing, field observations were conducted in Nanjing University of Information Science & Technology during the winters of 2006 and 2007, when 27 fog cases were acquired, including 22 cases of contiguous fog droplet spectrum data. According to the droplet spectrum distribution the radiation fog could be divided into wide-spectrum one, which is all extremely dense fog with the maximum diameter more than 40 μm , and narrow-spectrum one, most of which is dense fog with the maximum diameter less than 22 μm. During developing from dense fog to extremely dense fog, the radiation fog is characterized by the explosive deepening, whose nature is the fog droplet spectrum burst broadening, and the increase of number concentration and LWC. The macroscopic condition and microphysical process of the fog droplet spectrum burst broadening were detailedly analyzed in this paper, and the result shows that: the fog droplet spectrum burst broadening occurred during the period of temperature rapid decline (Super-saturation increase). The weak cold advection or the water evaporation from the wet surface after sunrise during the dense fog episode is apt to cause the burst broadening of the fog droplet spectrum. Early fog droplet spectrum broadening was mainly based on nucleation and condensation, but coalescence and condensation during the late period. The droplet spectrum “widening” has the characteristic of a ladder-type, but when the cooling rate and the degree of supersaturation are very large, the number of fog drops in all bins could increase, and the maximum diameter could enlarge continuously. The turbulence not only plays an important role in the vertical transmission of momentum, heat, and water vapor, but also is the important condition for fog droplet collision broadening.