The Jurassic Anding Formation serves as a critical aquifuge controlling roof water inrush from the Zhiluo Formation in the northeastern Ordos Basin. Taking the Mataihao Coal Mine in the Dongsheng coalfield as the study area, its impermeability performance and mechanism were systematically investigated through XRD, SEM, CT scanning, laser particle size analysis, permeability and hydrophysical property tests, combined with sedimentary characteristics. The results show that the Anding Formation is predominantly composed of sandy mudstone, mudstone, and siltstone. Quartz and muscovite form a rigid framework, while non-swelling clay minerals such as kaolinite and illite fill the pores, resulting in nanoscale pore throats with poor connectivity that create a “framework support–clay plugging” microstructure. The hydraulic conductivity is as low as 10?¹?–10?? cm/s. Upon water contact, the rock samples remain structurally stable; swelling and disintegration are governed by mud content, indicating favorable hydrophysical properties. The formation averages approximately 120 m in thickness, with a mean mud-to-sand ratio of 2.5, exhibiting good overall impermeability yet distinct spatial heterogeneity. This study reveals the mechanism by which microstructure and sedimentary characteristics jointly govern impermeability, offering a theoretical and engineering basis for roof water hazard prevention, control, and leakage risk assessment in mining areas.