Abstract: The productivity of coosystem is influenced by the regional wate conditions in a great degree. Theugh the various aridity coefficients may reflect the mean annual arid－humid condition, but they fail in revealing the dynamic temporal－spacial changing of water condition. This problem can be solved by the aridity line which is based on the water balance. Using the aridity line to analyse the regional water condition, we can subdivide the arid period (area), humid period (area) and determine the optimumrgrowing arca of crop. The analysing of the aridity line can be used as the foundation of irrigation, agricultural planning and distribution of agriculture.