Abstract: Poor sealing boreholes are the potential water-filling channels that induce mine water inrush accidents, and are the main hidden disaster factors in the Jurassic coalfields in the border area of Inner Mongolia and Shaanxi. For the deeply buried Jurassic coalfields in the border area of Inner Mongolia and Shaanxi, the buried depth is generally greater than 500 m, and during the mining period, the exposure of poor sealing borehole may communicate with the strong water-rich aquifer in the upper part of the coal seam, making it a direct water-filled water source for the coal seam, thus affecting the safe production of coal mines. In order to study the water output characteristics of poorly sealed boreholes in deep-buried Jurassic coal seams in the border area of Inner Mongolia and Shaanxi, a “trinity” accurate judgment method for underground water inrush sources was established, and the water inrush characteristics and water chemistry characteristics of boreholes were analyzed by comparing with typical drainage boreholes and water quality test data, and the hydraulic connection between poor sealing borehole and overlying aquifers on coal seams was analyzed by using a hierarchical cluster analysis model. Combined with the on-site test of the plugging effect of the poorly sealing borehole, the disaster-causing mechanism of the poorly sealing borehole was analyzed. The results show that there are great differences in the water quality characteristics of the Cretaceous pore-fissure aquifer system and the Jurassic fissured aquifer system in Inner Mongolia-Shaanxi border area, and the characteristic ion HCO₃⁻ of the Cretaceous pore-fissure aquifer system is HCO₃−Na type water, which accounts for more than 45% of the anion. The water sample of the Jurassic fractured aquifer system is of the SO₄−Na type, and its characteristic ion SO₄²⁻ accounts for more than 40% of the anion. At the same time, the water inrush characteristics of the downhole water inrush point are compared with the typical drainage water borehole to quickly identify the water-filling horizon, and the corresponding plugging effect test method is proposed, which can provide a certain reference for the emergency treatment of water inrush in the subsequent underground borehole with poor closure and the exploration of the plugging effect of the borehole.