Abstract: The interactions among the Asian—Pacific monsoon subsystems have significant impacts on the climatic regimes in the monsoon region and even the whole world. The Asian—Pacific summer monsoon system is under the control of the huge divergent circulation system which is divergent at the upper level and convergent at the lower level. Three branches of the air flows from the divergent center at the upper level play significant roles in the promotion of the Indian summer monsoon (ISM), the East Asian subtropical summer monsoon (EASM), and the South China Sea summer monsoon (SCSSM), respectively. It is one of the perfect performances of the holistic characteristics of the Asian—Pacific summer monsoon system. The leading modes of a couple of meteorological elements in the Asian—Pacific monsoon region have shown a more consistent characteristics on both the interdecadal and interannual timescales: the decreasing trend emerges in the Asian—Pacific summer monsoonon the interdecadal timescale during recent 55 years；while on the interannual timescale, the subsystems of the Asian-Pacific summer monsoon have synchronous variation with quasi-two-year and quasi-four-year cycles. The quasi-two-year oscillation of the monsoon system gives expression that when the ISM is anomalously strong, the monsoon rainfall belt will be strong and more northward, bringing more precipitation in the northern-central Indian continent. Because of the western Pacific subtropical high (WPSH)’s northward shift and the ISM’s anomalously eastward extension in the same year, an anomalous cyclone circulation occurs over the western North Pacific in 10°N-30°N and another anomalous anticyclone circulation in 30°N-50°N. It is indicated that the EASM is anomalously strong, and can push the monsoon belt northward to northern China. That is to say, when one of the Asian-Pacific summer monsoon subsystems is much stronger than normal, the other monsoon subsystem in the same year will be also stronger than normal, but both of them will become weaker than normal in the next year. The tropospheric biennial oscillation (TBO) may be an inherent cycle in the Asian-Pacific summer monsoon system. It reflects the significant holistic characteristics of the Asian-Pacific summer monsoon system on the interannual timescale under the response of the tropical Pacific—Indian Ocean temperature forcing.