Abstract: The mining of Xinwen Coalfield is affected by complex geological structures, and the threat of floor water hazards is particularly prominent. To explore the water inrush mechanism and disaster-causing characteristics of small faults in the lower coal seams of Xinwen Coalfield, this paper analyzes the characteristics of aquifers and water-conducting channels in the coalfield, summarizes typical cases of floor water inrush during the mining of lower coal seams, conducts an in-depth analysis on the activation and water inrush mechanism of small faults based on water inrush theories, and deduces the criterion for water inrush induced by small fault activation. With the aid of the FLAC^3D numerical model, the process of water inrush due to small fault activation during the mining of lower coal seams is simulated, and the water conduction process formed by the connection between the water-conducting damaged zone of the coal seam floor and the fault evolution zone is analyzed. The results show that: during working face mining, the floor rock strata undergo a dynamic process of compression-expansion-recovery, forming a floor water-conducting damaged zone and significantly reducing the water-resisting performance of the floor aquiclude; under the disturbance of mining activities, small floor faults are subjected to compression and shear effects, sliding failure occurs in the stress-relief zone, the fault evolution zone continuously expands, and eventually connects with the floor water-conducting damaged zone to form a complete water-conducting pathway; after faults enter the stress-relaxed zone, the total height H_s of confined water upwelling increases and the shear stress τ rises, leading to a continuous increase in water inrush risk with the advancement of the working face; as the mining distance advances gradually from 30 m to 120 m, the activation process of small floor faults experiences three temporal stages: initial disturbance, accelerated expansion, and connectivity instability, as well as three spatial stages: fault activation, connected water conduction, and connection of the “two zones”. The findings further verify the activation and water inrush mechanism of small faults during the mining of lower coal seams in the Xinwen Coalfield.