85-station daily precipitation data provided by the National Meteorological Information Center and NCEP/NCAR reanalysis data from 1961-2010 are used to select 2004 and 2010 as the serious drought and flood year of pre-flood precipitation over South China. The low-frequency characteristics of precipitation and cold air and their relationship are also be investigated. The main results are as follows:(1)The pre-flood season precipitation in flood year is mainly controlled by 10-20 d and 30-60 d oscillation. In drought year, 20-30 d is another significant low-frequency cycle. The low frequency potential vorticity has positive correlation with the same frequency precipitation, especially for 10-20 d low frequency potential vorticity. (2)For the flood year, the direct source of the 10-20 d low frequency cold air locates from the North China to the Bohai Sea area. Inclined 315 K isentropic surfaces from west Siberia region are the areas where vertical vorticity grows quickly most easily, it is also the path along which the upper cold air intrudes. For the drought year, there is little cold air transportation in the lower troposphere. 10-20 d low-frequency precipitation is influenced by the high potential vorticity pools which exists in upper troposphere (lower stratosphere) is the main source.