The relationship between the soil water content from the jointing of wheat to the heading and numbers of grains of every ear of wheat is the closest in the dry farming regions of the Loess Plateau. The relationship between the soil water content from the stages that wheat is in the milk to milky maturity and the weight of all grains of every ear is also the closest. Wheat is easily influenced by environment factors for two periods mentioned. The influence of the base soil water content before sowing of seeds on yield of wheat is evident. In the wake of wheat growing up, the soil water content in deep layer will play an important role.