<p>Wildfires enhance post-fire debris flow (PFDF) hazards by altering catchment hydrology and sediment dynamics, yet the mechanisms governing their spatiotemporal evolution during the first rainy season remain poorly constrained. Following the March 2024 Yajiang wildfire (Sichuan, China, ~ 278.81 km² burned), this study analyzes the first high-resolution PFDF inventory (506 events across 211 catchments) derived from field surveys and drone mapping. PFDF magnitude scales with catchment area, with narrow-steep catchments dominating activity due to amplified sediment connectivity and transport capacity. Temporal clustering of PFDFs occurred in May–July (79.6% of events), but the timing of initial debris flows varied systematically with catchment morphology: hillslope-incised systems initiated within 9 days post-fire (2.2 mm/h rainfall intensity), whereas wide-gentle catchments required 46 days (3.9 mm/h) in the Yajiang burned area. Divergent trajectories of activity intensity emerged as sediment sources evolved across different catchment geometry. Hillslope-incised catchments exhibited declining PFDF frequency as surficial materials were rapidly depleted, while narrow-steep erosional and wide-gentle systems intensified after 3–4 events due to sediment replenishment from gully erosion and bank failures. These shifts reflect a transition from hillslope-dominated to channel-coupled sediment cascades, modulated by morphological thresholds and rainfall seasonality. The results establish that PFDF hazards are not static but evolve dynamically during the first post-fire year, driven by feedbacks between sediment supply, catchment geometry, and hydroclimatic forcing. This study provides insights for post-fire landscape evolution and proposes mitigation strategies for PFDFs categorized by catchment types and temporal stages.</p>