A lastingseveralday heavy rain, which leads to severe floods and dis asters, occurred from June 17 to 24, 2005, when there exists strong cold air act ivity in the Southern Hemisphere. In order to investigate the effects of cold ai r from the Southern Hemisphere on the continuous heavy rain and its related phys ical way, taking the continuous heavy rain process as an example, the role of th e cold air in the formation of heavy rain is preliminarily analyzed with NCEP 6  hourly reanalysis data, hourly TBB and rainfall data in South China. The results are as follows: The Mascarene high moves eastward and lands in Australia under the steering of 500hPa westerly trough, which leads to the outbreak of cold air in the Southern Hemisphere and strengthens the crossequatorial flows at the longitudes of 40～60°E, 60～70°E and 85～95°E. Then the crossequatorial flows move toward South China, enhance the low l evel jet in the southern part of South China and the north of South China Sea an d increase the moisture transportation in heavy rain area. It is one of the main reasons in the formation of lastingseveralday heavy rains.