Abstract: In order to explore the interaction between the East Asian summer monsoon (EASM) and the South Asian summer monsoon (SASM) and its influence on summer precipitation in Southwest China, daily precipitation observations collected at 161 stations in Southwest China from 1979 to 2019 and the global reanalysis data of ERA-5 from 1979 to 2019 are used to compared the correlations of the standardized precipitation index in Southwest China with the intensity indexes of the East Asian and South Asian summer monsoons. Four types of monsoon synergy between EASM and SASM are proposed, and the influences of the four types of monsoon synergy on the precipitation in Southwest China are analyzed. The results show that: (1) There are four types of monsoon synergy in EASM and SASM: Strong EASM-strong SASM, strong EASM-weak SASM, weak EASM-weak SASM and weak EASM-strong SASM. The corresponding cooperative annual precipitation characteristics are the western Sichuan basin type, the whole Southwest consistent type, the whole Sichuan basin type and the eastern Southwest type. (2) In strong EASM-strong SASM years, the western Pacific subtropical high is weaker than normal and shifts to the east, the Iranian high is weaker than normal ad shifts to the west, and there are two cyclonic circulations located in the northeast of the Indian Peninsula and the South China Sea, respectively. The EASM transports water vapor from the South China Sea and the western Pacific to Southwest China. The water vapor convergence in Southwest China is weak with more descending motions and less precipitation. However, there exist obvious water vapor convergence and ascending motions in Chengdu Plain, leading to more precipitation there. In the strong EASM-weak SASM years, the western Pacific subtropical high is weaker and shifts to the east, and the Iranian high is stronger and located to the east of its normal position. The anticyclonic circulation and cyclonic circulation are located in the south of the Indian Peninsula and the western Pacific, respectively. The EASM transports water vapor from the South China Sea and the western Pacific to Southwest China. Significant water vapor convergence and upward movement develop in the southwest, resulting in more precipitation. In weak EASM-weak SASM years, the western Pacific subtropical high extends westward and connects with the Iranian high that extends eastward, while there is no obvious circulation loop in the low latitudes. Water vapor in the west of the Bengal Bay is transported northward to Sichuan Basin, accompanied by obvious upward movements. In other areas, water vapor is divergent with sinking air flow, and precipitation is less than normal. In the weak EASM-strong SASM years, the situation is basically opposite to that in the strong EASM-weak SASM years.