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This paper focuses on the risk of water inrush from the old goaf water of the upper coal group to the first mining face of the lower coal group under the condition of fully mechanized caving mining. Data from the 2101 working face of the No. 15 coal seam in the Wulihou Coal Mine were used to calculate the height of the water-conducting fracture zone on the first mining working face and the floor failure depth in the overlying No. 5 coal seam. An engineering geological model was developed, considering the lithological combination of the upper and lower coal groups, as well as the geological and hydrogeological conditions. Numerical simulations were conducted to analyse the deformation and failure characteristics of the roof and floor after dual-seam mining. The results revealed a floor damage depth of 12 m and a water-conducting fracture zone height of 109 m. Field monitoring data confirmed that after mining the 2101 working face, there was no water inrush risk from the overlying old goaf water into the No. 15 coal seam through the water-conducting fracture zone. The research results provide a reference for preventing water inrush accidents in similar fully mechanized top coal caving mining under the old goaf water. [ABSTRACT FROM AUTHOR]