Abstract: By using the routine sounding, surface observation data and NCEP reanalysis data, the comparative analysis resulting from moisture source and transfer, moisture budget and moisture flux of the five flooding rain occurred in the Weihe River valley in 2003 show that there are mainly two vapor sources in the flooding heavy rains in Weihe River, one is from the Gulf of Bangladesh, where water vapors transfer to Weihe River valley at 850—500 hPa, and the other is the South Sea, where water vapors transfer to Weihe River valley from surface to 850 hPa. In the continuous overcast and rainy days, though there are weak vapor flux convergence and transportation at low level, and only at high level there exist plentiful vapor convergence and vapor flux transportation, plentiful vapors accumulates in Weihe River valley resulting in heavy rains in Weihe River valley. The vapor channel of the flooding heavy rains has a good correlation with the west Pacific Ocean subtropical high position. When the west pacific ocean subtropical high strengthens westward or retreats eastward, the vapor channel position of the flooding heavy rains also moves westward or eastward and results in the heavy rain at upper or lower reaches of Weihe River. When the heavy rains happen, the specific humidity in precipitation area is more than 7 g/kg. In the verticality structure, at the high level, specific humidity rises quickly at first in the heavy rain area 6—12 hours before the heavy rain, then from the upper to the low level, specific humidity rises; and the rates of the specific humidity increase and decrease at high or low level are equivalent. When the specific humidity decreases to or lower than the initial value, the heavy rain stops. It is very important to the heavy rain storm forecast, because the Qinling mountains and Pamier plateau countercheck the vapor from south side, the vapor mostly imports to Weihe valley from south side at 700 hPa to 500 hPa level, this is very different from Huaihe River valley that the vapor mainly comes from 850 hPa to 700 hPa. At the same time vapor reaches Weihe River valley from a long journey which makes the vapor content decreases quickly. Comparing Huaihe River with Changjiang River valley, in the heavy rain storm in Weihe River valley, vapor content is significantly little at low level, and the precipitation intensity is weaker and the corresponding precipitation area is small. The vapor is mostly transferred from the west side. On the other hand, the east side is also an import side, water vapor at the east ocean from the east side are being transferred to Weihe River valley, but this situation is associated with the location of the west pacific ocean subtropical high. Water vapor advection only accounts for 6%to 30% on the whole vapor flux, it reflects that in heavy rain storms in Weihe River valley, vapor advection only have a little contribution and vapor mainly relays on the wind congregation to the heavy rain storm area. Precipitation has a good positive correlation with precipitable water, the bigger the precipitation course, the more the precipitable water quantity.The precipitable water quickly decrease means the heavy rain storm will soon stop.