Abstract: In this paper it is proved theoret ically that the variation ratio of regional mean water vapor flux can be divided into two components:the variation ratio of mathematical mean of regional mean water vapor flux for a region and,the disturbance variation ratio cont ributed by land surface heterogeneity,which is mainly influenced by variety coefficient of special distribution of soil moisture in the near surface soil layer in the region.It is demonstrated by numerical experiment that the variety coefficient of special distribution of soil moisture content can be used to denote the heterogeneity of soil water content in a spatial region,and the contribution of land surface heterogeneity of soil moisture to the variation ratio of regional mean water vapor flux are also different for diverse soil types.For the same mean of soil water content,the sensitivity of variation ratio of regional mean water vapor flux to land surface heterogeneity of soil moisture is dissimilar completely for different soil types.From this study it can be concluded that the heterogeneous soil moisture distribution on sub-grid scale region imposes import ant influence on estimation of grid-mean water vapor flux.The heterog eneity of soil moisture in near land surface layer on sub-grid scale should be taking into account in est imation of grid mean water vapor flux in land surface process model.The connotative un-realistic hypothesis of homogeneous soil moisture on gridscale region may make un-neglect able error for the est imations of regional mean water vapor flux.