The field experiments were conducted during a heavy fog event occurred in Tianjing on 30 November and 1 December 2009. The corresponding boundary layer and microphysical structure for the fog event are analyzed. Liquid Nitrogen (LN) was seeded into the heavy fog, and significant response was found after seeding. The results show that: cooling near the ground, warm advection of water vapor transmission, and deep inversion are the main causes of the strong fog. The broadening of droplet spectral is of explosive development characteristic. Average spectrum peaked at 6, 14, and 22 μm in diameter, respectively. Liquid nitrogen was seeded into the heavy fog, and the fog droplet spectral peak appeared near 11 μm. The number concentration increased significantly in the diameter range of 8 to 21 μm, with the order of magnitude of about 10 times higher, which is related to the ice crystal effect of liquid nitrogen.