Abstract: Detailed process and mechanisms of the abnormally early onset of the South China Sea Summer Monsoon (SCSSM) in 2019 were analyzed using the NCEP/NCAR reanalysis daily data and NOAA daily OLR and SSTA data. The results are as follows: (1) The SCSSM broke out on 6 May. The heat sources of Qinghai-Tibet Plateau and Indochina Peninsula are weaker than normal, and have little influence on the onset of SCSSM. (2) During the medium-range evolution of the atmospheric circulation, a warm high-pressure ridge formed and persisted for two weeks over the Tibetan Plateau and nearby. Under the influence of strong warm advection from the tropics and strong clear sky radiation in the eastern Qinghai-Tibet Plateau, this high-pressure ridge caused an abnormally high temperature in the middle-upper levels, which played a key role in the SCSSM onset by accelerating the process of seasonal change of land sea thermal difference. (3) The latent heat of condensation released by "Fani" during it northward movement and its residual low pressure after landing at northeastern Indian, the warm advection caused by the southwest monsoon in the Bengal that was triggered by condensational latent heat and the cyclonic circulation due to "Fani", the warm advection by the anticyclone circulation of the South Asian High (SAH) which was strengthened by "Fani", the condensational latent heat caused by convergence of southwest monsoon and backflow related to the East Asian cold trough after "Fani", step by step, formed a "relay" to strengthen the positive temperature anomaly, which eventually led to the increase of temperature gradient in the SCS, and resulted in the strengthening of the cross equatorial flows and the onset of SCSSM. (4) El Niño and periodic development of positive real-time SSTA in the equatorial western Pacific made the WNSPH stronger and located more westward, which is not favorable for the SCSSM onset. However, the easterly winds at the western WNSPH converged with northerly winds from the middle-high latitudes and formed a cyclone which grew dramatically under the background of strong positive SSTA in the equatorial Indian Ocean and the Bay of Bengal and produced "Fani", which developed to a typhoon and led to a series of circulation changes, and finally resulted in the extremely early onset of SCSSM.