Abstract: In order to solve the problem of mining safety under water pressure of aquifer in lower coal group of Zhuozishan coalfield in Inner Mongolia, so based on years of research – “Top of Ordovician exists carbonate aquifuge in North China”, Baiyunwusu mine which is nearby Luotuoshan mine is chosen as the field test in-situ, to carry out the macro- and micro-researches on the floor of lower coal seam (16#) and test mining. The results show that: 1) There is a layer of carbonate aquifuge , with 150 m in thickness, existed in the top of Ordovician underlying coal seams floor, while the lithology is dark gray marl with less mudstone deposition. Meanwhile, the real section of the aquifer is appeared in caesious dolomitization of limestone under-laied the dark gray marl strata; 2) The vast majority of pore  diameter of dark gray marl in Kelimoli group is less than 5 μm. Because of the poor pore connectivity, the effective pore is difficult to be formed and it is hard to seepage in nature; 3) The performance of the new tectonic movement is the resurrection of large old fault and generation of some small new fractures. Besides, the fractures provide the initial channel to Karst development and also constitute the main Karst water reservoir shift space and access; 4) The main threaten of Ordovician Karstic water to safety mining under water pressure of aquifer in Zhuozishan coalfield is hydraulic conductivity fracture, collapse column and other parts of the vertical channel, and the key area to control water inrush is Heilonggui thrust fault and F14 normal fault. Eventually, the safety mining in many workfaces in Baiyunwusu mine confirms that the top of Ordovician carbonatite strata is not only the aquifuge, but also the key aquifuge.