Abstract: In the context of NCEP/NCAR reanalysis and observations, study is undertaken of the causes of a heavy rain formation over the subtropical high center from July 26 to 27, 2003. Results show that cold air from the northeast at the mid-lower levels invading the high’s central region acts as an important factor, which functions in the following ways. First, the cold air leads to temperature drop in the middle to lower troposphere, with temperature rise in the boundary layer, thus causing unstable energy to be accumulated in large amounts over the fallout area. Second, it makes the boundary-layer wind converge, which thus excites ascending motion throughout the extent. Finally, this cold air provides rich supplies of water vapor for the rainstorm. Besides, the main body of the South Asia high moves east suddenly before the rainstorm begins and the axis of upper westerly jet keeps moving eastward and strengthening, with its exit region expanding towards the south, thereby shifting westward the main part of the central region of the high in the middle troposphere. In the meanwhile, the invading cold air is responsible for southward displacement of a cold vortex around Japan, creating a favorable condition for cold air to enter the rain zone in mid and lower levels.