The mean-years annual precipitation in three areas, the national range,Xinjiang Autonomy and the Urumchi River valley was analysed. The result shows that there is a functional relationship between the mean-years annual precipitation and its area,the relationship is a negative exponential distribution. The method can be used to (1) describe quantitatively the precipitation state in a climatic region ,and (2) estimate the mean-years annual precipitation in the regions without rainfall station by the flow-off of the river water and calculate some hydrometeorological parameters.