There are three main factors controlling the formation of debris flow; of these, the ability to evaluate the volume of source materials in a catchment is the most significant. Source materials come from channel bed sediment, nearby landslides and rilling and surface erosion of slopes. The objective of this study was to develop a multi-source method–including field surveys, optical remote sensing interpretation, and interferometric synthetic aperture radar (InSAR) technology–to estimate the volume of source materials in the debris flow in the Xulong Gully (XLG), China. The qualitative degree of stability of the source materials was estimated with volume of approximately 91.9 × 10⁴ m³. Considering sediment connectivity, landslides debris were interpreted using optical remote sensing, and their volume was calculated, using an empirical formula, to be about 191.01 × 10⁴ m³. Continuous monitoring using InSAR could help to obtain the large-scope precise process of ground surface deformation. Estimated erosion rate ranges from 1633 m³/(km²·year) to 4552 m³/(km²·year) and annual volume of erosion was 9.08 × 10⁴ m³/year–25.31 × 10⁴ m³/year. Higher elevation with good vegetation coverage showed the sedimentation process, while lower elevation area with little vegetation showed erosion process. The highest degree of erosion occurred in the summer, followed by spring, autumn, and winter. The trend of the degree of erosion was consistent with that of the monthly rainfall in the XLG in 2018. Verification results demonstrated that the proposed approach could improve the efficiency and accuracy of the estimates of source material volume in debris flows and assess hazards.