Surface water and groundwater interaction variations in time and space are crucial for effective water management, especially in low-precipitation regions. To comprehensively determine the hydrochemical characteristics and interaction processes of surface water and groundwater and to investigate the decreasing causes of water resources in semi-arid mountainous watersheds under changing environments, intensive field surveys were conducted in the Daqing River watershed, a tributary of the Haihe River basin in northern China, during two different times of the year: after the rainy season (September 2018) and before the rainy season (July 2019). Sixty surface water and groundwater samples were collected along the mountainous watershed. Using a combination method of hydrogen and oxygen stable isotope tracing and hydrochemical analysis, the hydrogen and oxygen isotopes and hydrochemical characteristics of surface water and groundwater in the mountainous watershed of the Daqing River were analyzed. Furthermore, the effect of elevation (altitude) on isotopes was discussed, and the correlation between hydrogen and oxygen isotope composition and hydrochemical characteristics was obtained. The results were processed using endmember mixing analysis to determine the amount of contribution of the surface water and groundwater interaction processes. The results show that the hydrochemical characteristics are relatively stable in the mountainous watersheds of the Daqing River, and the surface water and groundwater are mainly of the HCO3-Ca type. The slope of the local meteoric water line is smaller than the slope of the global meteoric water line, and the delta D and delta O-18 in surface water and groundwater show a good linear relationship both before and after the rainy season. There is a decreasing trend of the value of delta O-18 in surface water samples with decreasing altitude, but a decreasing trend of the value of delta O-18 in groundwater samples is not obvious. The evaporation intensity of surface water is stronger after the rainy season than before the rainy season, and the connection between the surface water and the groundwater is stronger before the rainy season. Influenced by topographic conditions and other factors, the exchange of surface water and groundwater is frequent, and there is a large difference in the exchange ratio before and after the rainy season. The exchange ratio can be more than 50% after the rainy season. Thus, the reasons for decreasing water resources in the mountains can be implied to be due to the increasing hydraulic gradient between the mountains and the piedmont plains, and the water resources are discharged more in the form of groundwater to the downstream. The conclusions help to enhance the understanding of the water cycle in the mountainous watershed and can provide some theoretical basis for the sustainable development and utilization of water resources in the Haihe River basin and the regional water ecology of the Xiong'an New Area.