Adopting NCAR/NCEP reanalysis data and APHRO_MA_V1003R1 precipitation data, this paper compares and analyzes the similarities and differences of climate features and impacts on precipitation distribution between the Southwest Summer Monsoon (SSM) of Bay of Bengal (BOB) and the South China Sea Summer Monsoon (SCSSM). The results show that: (1) Compared with SCSSM, the SSM of BOB establishes earlier and more intense, drives more northward, and its duration is longer. (2) The SSM of BOB establishes slowly because the Somali cross-equatorial flow is strengthened slowly and the Tropical Indian ITCZ (Inter-Tropical Convergence Zone) moves northward gradually. Whereas the SCSSM establishes quickly because of one time replacing process of East Asia Trough with sudden eastward enhancement of Western Pacific Subtropical High (WPSH) and sudden northward jump of Tropical Western Pacific ITCZ. (3) The zonal characteristics of BOB is stronger, and the main variation after monsoon establishment is the southwest wind strength; but the meridional characteristics of SCSSM are stronger, reversal of wind direction exists after monsoon establishment, and the southeast wind is replaced quickly by southwest wind because of eastward enhancement of WPSH. (4) The enhancement of SSM of BOB is quick while that of SCSSM is slow. (5) According to the monsoon process, Summer Monsoon (SM) period is divided into following stages: development (May), strengthening (June-August), and declining (September-October). In different periods, the development and movement of active convection areas, the positions of monsoon runs through and the corresponding precipitation areas have obvious differences. (6) In the SM period, monsoon meridional sub-circulations exist along the longitudes of BOB and SCS, respectively, which are distributed on south and north sides symmetrically, caused by northward movement of ITCZ, and ascend in the active convection region of monsoon trough and descends on south and north sides. Because of strong heat sources of BOB and the Tibetan Plateau, the meridional ascending span region of BOB is wider with north descending region north of the Plateau, while the descending region of SCS is more southerly. The meridional circulation of BOB is more stable and the meridional circulation of SCS swings in meridional direction is more obvious. The ascending center region of BOB is more northerly than that of SCS. In the decreasing period of SM, the ascending center region of BOB is more southerly than that of SCS, because ITCZ of SCS enhances slowly.
