Abstract: To obtain the radio interference (RI) levels and characteristics of high-altitude AC transmission lines in China, an observation station was established on a 750 kV AC transmission line in the Qinghai-Tibet plateau at an altitude of 3 320 m. A two-year long-term RI observation was conducted, collecting 920 000 valid data sets. The measurement data for each year were classified into six weather categories, namely, fair weather, heavy rain, light to moderate rain, heavy snow, light to moderate snow, and haze weather, and statistical results for each weather condition were obtained. The study shows that the average RI at the Gangcha observation station during heavy rain and fair weather is 69.09 dB and 49.22 dB, respectively. The probability of RI being below 58 dB across all weather conditions is 95%, meeting the limit requirements. The difference in average RI for the same weather conditions across different years is minimal, and a one-year observation period is sufficient for determining the long-term RI levels of AC transmission lines. Furthermore, a method was proposed to quickly obtain the approximate average value of RI during heavy rain by conducting short-term observations during periods of heavy rain or snow, comparable to the average RI obtained from one year of long-term testing. This method can be utilized to assess the RI levels of transmission lines, significantly reducing costs and improving efficiency. The RI prediction results based on IEC/CISPR standards are approximately 4 dB higher than the measured values, thus the prediction methods for RI in high-altitude transmission lines need to be further researched.