Abstract: Based on NCEP/NCAR gridded reanalysis, TRMM precipitation data, CMAP, and rainfall observations in East China, a study is conducted with focus on the timing and distinctive establishment of the rainy season of the East Asian subtropical monsoon (EASM) in relation to the South China Sea (SCS) tropical summer monsoon (SCSM). A possible mechanism for the EASM is investigated. The results suggest that (1) the EASM rainy season begins at first over the south of the Jiangnan region to the north of South China in late March to early April (i.e., pentads 16-18), and then the early flooding period in South China starts when southerly winds enhance and convective rainfall increases pronouncedly; (2) the establishment of the EASM rainy season is earlier than that of its counterpart, the SCSM. The EASM and the SCSM each is featured with its own independent rain belt, strong southwesterly wind, intense vertical motion, and robust Low-level water vapor convergence. The SCSM interacts with the EASM, causing the EASM rainy belt to move northward. The two systems are responsible for the floods/droughts over the eastern China; and (3) in mid-late March, the eastern Asian landmass (especially the Tibetan Plateau) has its thermal condition changing from a cold to a heat source for the atmosphere. A reversal of the zonal thermal contrast and related temperature and pressure contrasts between the landmass and the western Pacific happens. The argument about whether or not the dynamic and thermal effects of the landmass really act as a mechanism for the earlier establishment of the EASM rain belt is discussed and to be further clarified. Finally, the article presents some common understandings and disagreements regarding the EASM.