Using the historical observation data and NCEP reanalysis data, an investigation is conducted on 31 returnflow events occurred over North China. On the basis of the 500 hPa circulation patterns, 31 events are divided into two types. The dynamical conditions and water vapor sources of these weather events, such as vorticity, divergence, vertical velocity, etc, are diagnosed. The results show that: (1) the dynamical conditions for returnflow events are a little different from other rainfall events, with divergence and anticyclonic circulation at lower levels; (2) the air of the returnflow from Northeast China, through the Bohai Sea, then to North China, is relatively dry.