Abstract: By decomposing the water vapor transport vector into the sum of its nondivergent(rotational) and divergent(irrotational) components in terms of the streamfunction and potential, the water vapor background of the 1998 severe flood is investigated over the globe.Then, based on precipitation analysis, 7 precipitation periods and 11 regions are divided during May-August 1998 the severe flood periods over Changjiang and Songhua River Valley.After water budgets of all precipitation periods and regions are diagnosed, some schematics of water vapor budget are gained, the main results are as follows:(1) The East China is the strongest water vapor sink over the globe in May-August 1998.The minimum potential region(maximum convergent region) of water vapor transport vector corresponds to strong rainfall region, and the water vapor convergence of the heavy rain region is made by the hemisphere scale water vapor transportation, indicating that to regional severe flood it must gain water vapor from very large regions.The South China Sea monsoon breaking out and it's south west water vapor flux becoming stronger are related to the potential being stronger in Indian Ocean.(2) At mospheric water vapor budget indicates that precipitation comes mainly from the convergence of water vapor and the convergence mainly in the lower at mosphere; The local evaporation(calculated by the residual) is generally 13~12 of the precipitation, so the water vapor recycle is also important; The vertical advection of the water vapor transports the moist urefrom the low to the middle and high layers and increases the high level moisture content, thus provides condition for the cumulus development and latent heat release.(3) The water vapor transportation in South China Sea is well related to the China strong rainfall.After South China Sea monsoon breaking out, the strong precipitation often occurs in the region whose water vapor gained by water vapor transportation from south.To East China, the water vapor from south boundary is more than from the west.(4) After South China Sea monsoon breaking out, the water vapor from west boundary through Indo-China Peninsula to South China Sea increases and becomes main water vapor source, therefore an obvious source is formed in South China Sea and the accumulative water vapor turns to north and transports to South China and Changjiang River Valley and affords t he water vapor condition for their strong rainfall.(5) The strong precipitation process during the Changjiang River Valley severe flood is well related to the max imum water vapor convergence, and the strong or weak precipitation is consistent with water vapor convergence, and this indicates that the latter is a necessary condition of the former.