The Binggou River Basin in the eastern part of the Qilian Mountains, located at the edge of a monsoon region, is the research object of this paper. The isotope technique and HYSPLIT 4.9 model were used to study the variation characteristics of the stable isotopes of precipitation and the source of water vapor in this area. The results show that the slope and intercept of the meteoric water line in the Binggou River Basin are significantly lower than those of the global meteoric water line and slightly lower than those of the meteoric water line in northwest China, and the slope and intercept of the meteoric water line in summer and autumn are obviously smaller than they are in winter and spring. The δ¹⁸O of precipitation is enriched in the warm season, and this obvious temperature effect (greater than 0ºC) masked the embodiment of the precipitation effect to some extent. There are great differences in the circulation systems that control the water vapor sources of precipitation in different seasons, and the westerly circulation that carries the water vapor has the greatest impact on Binggou River Basin precipitation, with an average proportion of 53%. The southwestern monsoon and southeastern monsoon also influence the summer precipitation. When the water vapor from the surrounding oceans and lakes reaches the monsoon marginal area in different seasons, the properties of the air mass change significantly, showing a significant seasonal effect on the precipitation value of δ18O and the δD. The seasonal fluctuation characteristics of the stable isotopes of precipitation and their relationship with the temperature, as well as the complex water vapor recharge sources in the Binggou River Basin, show the characteristics of a transition from the eastern monsoon region to the northwestern arid region.