Yunnan plays a pivotal role in transmitting electricity from west to east within China's Southern Power Grid. During January 7-13, 2021, a large-scale continuous ice-covering event of ultra-high voltage (UHV) transmission lines occurred in Qujing area in eastern Yunnan Province. Based on ERA5 reanalysis data and meteorological observation data of UHV transmission line icing in China Southern Power Grid, the synoptic causes of the icing are analyzed comprehensively from the aspects of weather situation, vertical stratification of temperature and humidity, local meteorological elements, and atmospheric circulation indices. The results show that the East Asian trough is strong, and the blocking high leads the northern air flow in front of the ridge to the south, and the cold air enters the ridge and joins the warm and wet air brought by the subtropical high pressure of 50-90°E. When the warm and cold air masses confront each other to form a quasi-stationary front over the northern mountains of Qujing, the alternating dominance of the warm and cold air is the main reason for the repeated ice accumulation of the power lines. Indices such as the Siberian High, East Asian Trough, and 50-90 ° E Subtropical High provided clear precursor signals within 0-2 days in advance of the icing events.