Abstract: Guangxi Province is important for China Southern Power Grid to transmit power from west to east. From January 26 to February 8, 2015, a large scale and continuous conductor icing event of ultrahigh-voltage(UHV) transmission lines occurred in Guilin, Guangxi, with a maximum icing thickness of24.83 mm. Using ERA5 reanalysis data and meteorological observational data combined with the icing observational data of UHV transmission lines from China Southern Power Grid, the current research comprehensively analyzes the synoptic cause of this conductor icing event with a focus on synoptic situation, vertical stratification of temperature and humidity, local meteorological elements, and atmospheric circulation index. Resultsshow that when the East Asian trough is strong, the blocking high pressure guides the northerly cold airflow from the front of the ridge to the south, and the cold air merges with the warm and humid air flow brought by the western Pacific subtropical high. The transmission line towers located in the high altitude are prone to freezing under the joint influence of the cold wave from the north and the water vapor from the south. There are two opposite changes in conductor icing when the cold and warm air confront over the mountainous areas in northern Guilin, forming a quasi-stationary front. When the cold air is strong and moisture is abundant, the supercooled raindrops freeze or the fog droplets condensate to form conductor icing. However, when the warm air dominates, conductor icing melts spontaneously. The movement of the front area significantly affects the growth of the icing of the wire; in particularly, the alternating dominance of the cold and warm air is the main cause of the repeated icing of the wire.