Eco environment is more fragile in arid and semiarid region. Especially in the condition of human irrational development, eco environment suffers serious threat. In order to save eco environment in arid and semiarid region in western China, our government made demand of "forest restoration from cropland". Scientificmanagement and protection of vegetation eco environment is very important to protecting eco environment against disruption and promoting the coordinated growth of soc economy system and eco environment system and building a harmonious relationship between human behavior and natural world, especially in the arid and semiarid region, western China.In view of arid and semiarid region, how to count and define actual water quantity used for vegetation ecological system and ecological use for water resources for some goal? It is very important in river basin program, eco environment construction and protection, water resources rational disposition. First, this paper introduces the concept of ecological use for water resources from the general meaning of ecological system, and then introduces its calculation method. This paper specially introduces the method of calculation of ecological use for water resources in case of "forest restoration from cropland" and "vegetation ecology improve and resume".