Through analysis of soil water and phenological data of four successive years, regular pattern of winter wheat evapotranspiration was analyzed for the area of Xifeng, Gansu Province and the critical period of water requirement was determined during which water plays the most important role in the development and yield formation of the crop. A notion, water consumption characteristic coefficient, was presented and used for evaluating the status of water supply and demand in various developmental stages. Results showed that lower yield might be caused by a smaller coefficient during the period from elongation to ripening. Therefore booting water is needed for reduction of sterile spikelet and grout filling water for increase of grain weight.