Abstract: Runoff component composition reflects the functioning and characteristics of catchment hydrological systems and insufficient understanding of runoff generation processes and their components has constrained the development of runoff generation theories. Therefore, this study investigates runoff generation processes and component composition in humid mountainous regions using a nested experimental observation system in the headwaters of the Taihu Lake Catchment. In-situ hydrological observations, isotope geochemical tracing, and digital filtering were integrated to analyze runoff composition characteristics and their responses to precipitation. Plot-scale observations indicate that surface runoff exceeds 50% of total runoff in both wet and dry years. In contrast, isotope tracing results reveal that quick flow contributes less than 50%. This discrepancy implies that steep terrains and shallow soils promote saturation-excess overland flow and interflow exfiltration, yielding a substantial contribution of old water to surface runoff. Due to the small catchment area and limited storage and attenuation capacity of channels and valley bottoms, runoff characteristics exhibit minimal scale effects. A unit increase in annual precipitation increases the depths of direct runoff (surface runoff plus interflow) and baseflow by 0.68 and 0.23 units, respectively, while greater flood season precipitation reduces the baseflow index. Overall, the proposed integrated analytical framework provides a comprehensive assessment of runoff component dynamics in humid catchments and offers a scientific basis for advancing runoff generation theory and prediction in mountainous catchments.