Abstract: Based on diagnostic results, a numerical study is made of the processes of Australian cold air activity affecting East Asian summer monsoon by using Kuo-Qian P-σ incorporated coordinate fivelayer primitive equation spherical band model. Analysis is done of the response to the Southern Hemisphere circulation with and without cold air activity in Australia of the flow, rainfall and diabatic heating fields in the monsoon area of Asia, especially, East Asia, with special attention to the intensification and northward march of the monsoon due to the activity.It is found that the processes for the effect transmission are very analogous to the meridional propagation of quasi-40-day oscillation, together with the meridional wind disturbance showing south-north travel and the flow/rainfall fields exhibiting corresponding movement in this. direction, only with a 12-day lag.