Water supply and the conversion relationship among different water sources play an important role in affecting wetland hydrological processes and the associated ecological functions. Under the guidance of the national strategy of ecological protection and high-quality development of the Yellow River, exploring the conversion relationship among rainfall, surface water, and groundwater in the estuarine wetland of the Yellow River Basin is the key to reveal the mechanism of regional wetland system evolution and the water conversion processes in the arid and semi-arid area. In this study, we aim to investigate the characteristics of hydrogen and oxygen isotopes of rainfall, river water, groundwater, soil water in the Fenhe River estuarine wetland (the Yellow River) in May, July, and September during 2019. To achieve these objectives, the water sources composition of wetland in different hydrological periods was analyzed. In addition, the transformation relationship among rainfall, surface water, and groundwater was quantified using a mixed source model. The results show that the isotope composition of various water sources is distinctly different. The mean values of δD and δ¹⁸O for different water sources are in the order of rainfall, soil water, Fenhe River water, watershed groundwater, and the Yellow River water, demonstrating that the discharge of the Yellow River water and the watershed groundwater plays a prominent role in maintaining the downstream runoffs of Fenhe River and the wetland water sources. During the drought season, the wetland groundwater in the riparian zone is mainly supplied by Fenhe River. Because of the increased distance to Fenhe River, the wetland is mainly recharged by the shallow groundwater from the Fenhe River basin, and then discharges to the Yellow River. While the Yellow River water transformed to the wetland groundwater in high water level season. The wetland soil water is mainly recharged by the local rainfall and the wetland groundwater. The transformation relationship among rainfall, river water, and groundwater varies between different seasons. During the drought season, the shallow groundwater in Fenhe River basin discharges into the Fenhe River, with a contribution proportion of 78%. During the flood period, the Yellow River acts to restrict discharge from the Fenhe River, and the flow reversals occur as the Yellow River's strong blocking effect. Consequently, the watershed shallow groundwater is generally replenished by the Fenhe River water at high water level stage, and the proportions can reach up to approximately 69%. However, the Yellow River water, wetland groundwater, and the shallow watershed groundwater discharge to the surrounding Fenhe River channel during the falling water period. The outcomes of this paper are of great significance for national ecological conservation of the Yellow River Basin and the restoration of natural wetland, contributing to further understanding water resources composition and evolution mechanism in other similar estuarine wetlands.