Abstract: Two typical debris flow gullies in Hot Water River Basin, Laowa Gully and Fencha Gully are taken as research objects in this paper. The initiation mechanism of debris flow hazard is revealed by means of field engineering geological investigation and analysis of the formation and development conditions of debris flow. The topography of the debris flow formation areas is steep and its catchment areas are fan-shaped. There are rich loose deposits as source conditions in the gully slopes and trench beds. The abundant rainfall and hydrodynamic conditions in rainy season provide a good water source for the formation and development of debris flows, which causes the debris flow hazard. The efficiency of different debris flow hazard mitigation engineering is evaluated using the matrix discrete element numerical simulation software MatDEM. Specifically, 62.3% of the total energy carried by the debris flow source is dissipated as system heat energy and 37.7% is beared by the dam structure under the geotechnical engineering measures. While 70% of the total energy carried by the debris flow source is consumed by ecological engineering measures and the other 30% is beared by the dam structure under the ecological engineering and geotechnical engineering measures. Based on the above results, the debris flow prevention and mitigation schemes are proposed for these two typical debris flow gullies under the synergistic effect of ecological engineering and geotechnical engineering, which provides references for the establishment of debris flow hazard mitigation mode in small watershed in mountainous area.