Abstract: Based on the physical mechanism of hydrological process,a parameterization scheme for regional averaged evapotranspiration with statistical-dynamic theory is established in the moist climate region.The regional averaged evapotranspiration is supposed to be constituted of the evaporation from bare land surface and the evapotranspiration from vegetation surface.At the same time the influence of proportion of saturation and non-saturation area of surface soil humidity in the examination region on the estimation of the evaporation or the vapotranspiration is considered.A expression for estimating regional averaged evapotranspiration is educed by means of the probability density function of soil humidity over heterogeneous surface for moist climate region.The reliability and workability of the expression are proved in the Yangtze Delta region.