AbstractCoal mining operations in China are often threatened by the invasion of the groundwater into the mining panels. The most likely water disaster affecting the safe operation of coal mines is water from Cenozoic porous aquifers in the alluvial plain areas flooding into the mine. The problem will become very serious when the mining-induced water-conducting fractured zone penetrates upwards into the aquifers, since it provides access for water inflow into the mine working panels. Therefore, it is of crucial importance to determine water-conducting fractured zones for predicting and preventing flooding in coal mines during mining. However, most Chinese coal mines have little technical equipment for monitoring such thing or professional staff to use such equipment. Such situations are even more serious in small coal companies. So a simple, manually operated instrument that has the advantage of being intrinsically safe and can be operated in gassy underground coal mines is popular in China. The instrument, named Double-End Blocking (DEB) device, measures the injected water loss rate along the borehole while the borehole is blocked on both ends and water is pumped in it. Comparing injected water loss rate between pre- and post-mining, the water-conducting fractured zone enhancement can be detected because the water loss rate will be larger in the stratum where the water-conducting fracture happened. In this paper we used the instrument to detect overlying strata water-conducting fractured zone height in one coal mine, which operated beneath a water-bearing alluvium. The overlying Cenozoic porous aquifer is a deadly threat. The results show that the instrument provides a simple and efficient method to determine the water-conducting fractured zone.