Understanding the stable hydrogen and oxygen isotope composition of river water in the basin is of great significance for identifying the evolution mechanism of water cycle and the composition of water source. Based on the test results of δD and δ ¹⁸O of river water and precipitation samples collected from Shiyang River in August 2020, this paper analyzed the composition characteristics of δ ¹⁸O and δD of river water in Shiyang River Basin during rainy period, and further revealed the spatial evolution characteristics and influencing factors. The results show that the δ ¹⁸O and δD values of the main and tributaries of Shiyang River are gradually enriched along the river, showing an increasing trend, while the excess deuterium is decreasing along the river. The effect of δ ¹⁸O and δD in river water is significant, and the variation of δ ¹⁸O altitude gradient is smaller than that of local precipitation and global precipitation. Using the evaporation loss calculation model, it can be seen that evaporation fractionation has an obvious effect on the water isotope change of Shiyang River, and the main stream of Shiyang River has the greatest effect, followed by Jinta River and Zamu River. The evaporation of reservoir water is more intense than that of river water. Precipitation is the main source of river water, and the isotope is mainly affected by the isotopic composition of river water in tributaries, and is controlled by the elevation effect, the continuous evaporation effect along the direction of runoff, and the higher degree of evaporation effect in the rainy season.