The dry farming area in the upper reaches of Weihe River in Gansu belongs to the semi-arid and semi-humid climate transition region. Soil moisture is the major factor affecting the growth of winter wheat in this region. Exploring the effect of soil moisture on the yield of winter wheat has great practical significance to ensure the food security production in dry farming areas. Based on the observation data of 0-50 cm soil moisture and yield of winter wheat experimental field from 1981 to 2020 in Tianshui Agrometeorological Experimental Station, which was a typical representative station, the variations of soil water and its effect on yield in 1981-2020 were calculated and analyzed by statistical method. The results showed that the variation trend of average soil water storage in 1981-2020 was not significant during the whole growth period of winter wheat in dry farming area in the upper reaches of Weihe River in Gansu. The period of maximum water storage was in the growth stage before winter, and the period of minimum water storage was from jointing stage to heading stage. The water storage showed a significant decreasing trend in 2010-2020 in the growth stage before winter of winter wheat, and it showed an increasing trend during the heading stage to maturity stage. The change of soil water content in each soil layer showed a sine wave trend with time, while the lowest soil water content in the plough layer was in early-May, and that in the deeper layer was in mid-May. The mid-June was the period with the lowest soil water storage during the whole growth period of winter wheat, and the accumulative dissipation was increasing fastest in this period. The thousand-grain weight and grouting speed of winter wheat in rainy years were generally higher and faster than those in dry years. Mid-July to early-September and early-Mar to mid-June were two most significant positive effect periods of soil water storage on yield, while the negative effect period was from late-June to early-July. The influence period of high altitude planting area is about 10 days later than that of low altitude area. The flowering and filling stage of winter wheat, which is late-May to mid-June, is the most affected period by water stress.