The Chuosijia Hydropower Station’s diversion tunnel encountered a fault zone during the excavation of the Sijiabishang Gully. This fault zone is accompanied by roof collapse and continuous debris flow formation due to water-rock mixing, making it difficult to install supports effectively and endangers tunnel safety. To identify fault zone size and spatial distribution, assess debris flow evolution mechanisms, and develop mitigation methods, an integrated technical approach comprising geological study, sophisticated seismic wave detection, and three-dimensional (3D) geological modeling is used. According to research findings, the steeply descending reversal fault f7-17 has hydraulic interaction with surface gullies. Excavation-induced disturbance weakens the fault’s water-resisting structure, allowing surface water infiltration and subsequent mixing with fault gouge to produce debris flow. By implementing comprehensive countermeasures including closed counter-pressure, adjacent section reinforcement, multi-layer frontal sealing, post-backfill grouting, and rock-water separation, debris flow discharge rate was effectively controlled from 15 m³/h to 0.5 m³/h, with maximum strain in support structures stabilizing. The proposed progressive prevention framework of “geological analysis-geophysical prediction-3D simulation-precision support” for continuous fault-induced debris flow management provides technical references for prediction and treatment of analogous geological hazards.