Abstract: In this paper, we put forward a mechanism about the Qausi Beweekly oscillations of South Asian monsoon system. Its main opinions are as follows :(1) In southern hemisphere, the oscillations of Australia high not only affect South China sea monsoon trough, but also affect the actions of Mascarene high by propagating westward. So it plays an important role in the oscillations of South Asia monsoon system.(2) In northern hemisphere, the oscillations of Indian monsoon trough propagate eastward to result in the South China sea monsoon trough, and sequentially, to cause the active break circle of the rain belt in Chinse continent, the oscillations of South Asia high and West Pacific high.