Based on the station observation, sounding data, FY-4B satellite images, Doppler weather radar data and ERA5 reanalysis data, the characteristics and causes of the anomalous change in the intensity of Super Typhoon Yagi in 2024 are analyzed. It is found that Typhoon Yagi is the stongest landfalling autumn typhoon in China since 1981, and it maintained super typhoon level (SuperTY) in the South China Sea for 64 h, making it the longest-lasting super typhoon in the region. Typhoon Yagi experienced three rapid intensification (RI) processes. In one of these processes, it reached the level of extreme rapid intensification (ERI), with its 12 h and 24 h intensity increase amplitudes being the highest in the South China Sea since 1981. The robust southwestern monsoon supplied abundant moisture for the typhoon development, while the stable subtropical high and continental airflow transported convectively unstable air masses from land toward the typhoon’s core, which enhanced the atmospheric potential instability near the typhoon circulation. Meanwhile, the weakened environmental vertical wind shear and favorable upper-level outflow conditions collectively facilitated the RI and the sustained peak intensity. The sea surface temperature in the northern South China Sea exceeded 30℃, 1-2℃ above normal, providing substantial latent heat flux to the atmosphere. During Yagi’s RI, a significant increase in kinetic energy within the typhoon’s domain was observed. The primary driver of this kinetic energy growth was a sharp rise in kinetic energy production. Moreover, the vertical and horizontal kinetic energy transports also contributed a lot.