The Chuosijia Hydropower Station's diversion tunnel encountered a fault zone during the excavation of the Sijiabi Upper Gully. This fault zone was accompanied by roof collapse and continuous debris flow formation due to water-rock mixing, which made it difficult to install supports effectively and endangered tunnel safety. In order 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 3D geological modeling was used. According to research findings, the steeply descending reversal fault f7–17 has hydraulic interaction with surface gullies. Excavation-induced disturbance weakened the fault's water-resisting structure, allowing surface water infiltration and subsequent mixing with fault gouge to produce debris flow. Through comprehensive countermeasures including “backpressure sealing, 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 εmax in support structures stabilizing from 7.2%. This study proposes a progressive prevention framework of “geological analysis-geophysical prediction-3D simulation-precision support” for continuous fault-induced debris flow management, providing technical references for prediction and treatment of analogous geological hazards.