Based on the evaluation of the drought in the south of the Yangtze river in the summer of 2003, the cause of the drought is analyzed. The result shows that in the same period of the drought, the polar vortex at 500hPa slant to the west hemisphere, in the middle and high latitude in Asia and Europe, there are two troughs and one ridge. The west wind front zone exists in the vicinity of 40°N, the west pacific subtropical high controls the south of the Yangtze river steadily for a long time.