Abstract: Based on the conventional meteorological observation data in Seni district of Nagqu, combined with the NCEP (National Center for Environmental Prediction, 1°×1°) and Era5 (0.25°×0.25°) reanalysis data, the formation mechanism, physical structure characteristics, and the cause of local occurrence of thick fog in north Tibet on April 10, 2019, were diagnosed and analyzed from the perspective of synopology. The results show that the fog is characterized by local outbreaks. The evaporation of water vapor from the melting snow in the early stage cooperated with the wind field is helpful to the formation of the fog. The 500 hPa circulation background configured with warm advection on the upper layer and cold advection on the lower layer in the boundary layer are involved in the formation of the fog so that the fog can be developed and maintained. There are fewer clouds at night, and the net radiation on the surface is strengthened. Strong net radiation on the ground, and a cooling effect leading to the saturation of water vapor, which is conducive to condense water vapor and form countless small droplets suspended in the air. The low wind speed, inverse temperature, and sinking motion in near formation make water vapor difficult to diffuse into the upper air, so large number of water vapor converges in a relatively limited space, leading to the explosive development of fog. The locality of fog is closely related to the special terrain.