Using the fog data of expressway in Sichuan and meteorological data from 2006 to 2013, the characteristics of dense fogs which made the expressway in Sichuan close and the corresponding causes are analyzed. The main conclusions are as follows: dense fogs leading to road closure mainly occur from November to the following January, mostly starting in the period of 05:00 to 08:00 in the early morning, usually ending from 08:00 to 11:00 in the morning, with the duration of road closure basically being 0 to 9 hours. There is generally an evenly distributed pressure field while dense fog occurs, and the low levels are controlled by high pressure circulation. The upperlevel circulation conditions of dense fogs can be classified into two patterns: northwest flow and straight westerly flow. The meteorological conditions beneficial to the formation of dense fogs include significant radiation cooling at night, weak wind and high humidity from the ground to the near surface layer, and temperature inversion at the surface layer.