In the boreal autumn of 2019, the equatorial Indian Ocean region experienced the most severe positive Indian Ocean Dipole (Indian Ocean Dipole, IOD) in nearly four decades. The event caused severe extreme climate disasters in Australia and East Africa. The sea temperature data from the Hadley Data Center of the UK and the wind field, geopotential height field and surface pressure data of the Fifth Generation European Centre for Medium-Range Weather Forecasts Atmospheric Reanalysis were used to explore the cause of the extreme positive IOD event in the boreal autumn of 2019. Results show that there was sufficient easterly flow in the equatorial Indian Ocean due to the combined action of the Australia High from June to July in 2019 and the cross-equatorial flow in the Bay of Bengal in August 2019. The extreme positive IOD event broke out in the boreal autumn of 2019 under the action of the Bjerknes positive feedback mechanism.The intensity of the Australia High from June to July and the cross-equatorial flow in the Bay of Bengal in August were independent of each other, and there was a significantly positive correlation between the intensity of the IOD event from September to November.In conclusion, when the Australia High from June to July and the cross-equatorial flow in the Bay of Bengal in August are both significantly stronger, an extreme positive IOD event will probably broke out from September to November in the same year,which will have serious impacts on agriculture, economy, and ecosystems of countries around the East and West Indian Oceans.