Abstract: A calculation method of subgrid scale runoff ratio over the heterogeneous surface is derived using the distribution theory of probability statistics on the basis of physics mechanism of water balance over land surface. The mean runoff and its out-growth and confluence over a grid region may be calculated when this method is used in mosaic scheme. It is demonstrated by test that due to difference of soil penet rability, even if the rainfall force is same for dif ferent soil types, the runoff charact eristic are also different. For any special scale, the surface heterogeneity of soil characteristic、land cover and climatic force are all exert influence on the het erogeneity of runoff. In this paper, the dependability and availability of this calculation method are verified from some example using actual observation data.