With the help of comparison between watersheds,we analysed quantitatively the main element of water balance:distribution among rainfall,evapotraspiration,interception and volume of runoff，the quantitative relationship among the elements and the affect by forest condition;hence we worked out a list of water balance. The result is : the rainfall to the water and soil prevension forest is chiefly distributed in evapotraspiration,interception of the tree crowns and on a small quantity of surface rainfall. Also we have this result;in order to make full use of the function of water prevension of westnorth in Shanxi,we must adopt kinds of methods to prevent the evaporation loss of water in the water and soil prevension forest,and make more water stored in soil. At the same time we offered the theoretical basis for planting and renewing the water and soil prevension forest.