Abstract: Based on the synoptic map and one-minute visibility data from automatic weather stations and traffic weather stations, the causes of persistent heavy fog occurred on 9 to 12 March 2017 on expressways of Guizhou were analyzed. The countermeasures of heavy fog weather on mountainous expressways were discussed. The results showed that heavy fog mainly occurred on the expressways under the influence of upper trough eastward movement, water vapor transports from the South China Sea to Guizhou by the southwest airflow, and the east-west shift and long-term existence of the static front system.Visibility in adjacent areas was negatively correlated with altitude because of the influence of frontal weather system. The fog was heavier and lasted for longer time at higher altitudes, resulting in lower visibility. Heavy fog was more likely to occur on expressways in Guizhou because of the relatively high altitude. Atmospheric stratification was stable under the frontal inversion, which was conducive to water vapor accumulation at low levels. The abundant water vapor and convergence was conducive to the development and thickening of low clouds and cloud bottom descent, resulting in the formation and maintenance of heavy fog near the ground. The stability layer was destroyed as the inversion layer weakened and disappeared. This was not conducive to water vapor accumulation and low cloud development, which was beneficial to the weakening and dissipating of the heavy fog. In order to deal with heavy fog weather, special attentions should be paid to the influence of contrasting visibility between inside and outside of tunnels, as well as the influence of wet road surface and low visibility in frontal heavy fog.