Abstract: Height of water-conducting zone is an important parameter for implementing water preserved mining techni- cal measures. In order to obtain the height of water-conducting zone in longwall top-coal mining caving mining face un- der thick alluvium and soft overburden,taking the 11915 working face of a mine as an example,three method were used to observe the height of water-conducting zone including borehole water leakage,borehole TV and numerical sim- ulation. Based on the observation of water leakage in three boreholes constructed underground,the variation law of wa- ter leakage was determined in boreholes before and after mining. The height of water-conducting zone was finally deter- mined according to the characteristics of borehole wall fissures observed by borehole TV,and the development law of water-conducting zone was analyzed by numerical simulation. The study results show that the height of water-conduc- ting zone is 45. 7-46. 7 m,the height of caving zone is 16. 1 m. The fracture-mining ratio is 6. 87-7. 02 and the ca- ving-mining ratio is 2. 42 in longwall top-coal mining under thick coal seam (6. 65 m),and soft rock stratum and thick alluvium (102 m). The height of water-conducting zone obtained by numerical simulation is consistent with the field measurement results. The development of water-conducting fracture zone has experienced four stages:development, slow increase,sudden increase and stability. Fractured zone develops upward in the form of separation layer and exists in articulated rock block type. The borehole TV can directly and clearly acquire the development characteristics of o- verburden fracture,and the best method to detect the height of water-conducting zone is borehole method. It is reliable to use borehole leakage method to observe the height of water-conducting zone. However,the observation equipment needs to satisfy a certain sealing pressure,and the sufficient length of sealing capsule. Also,the contact state between the capsule and the pore wall needs to be improved. In general,borehole method is the most reliable method to observe the height of water-conducting zone at present,but it needs a large amount of construction work and has a high cost. Currently,geophysical prospecting method should be improved on the sensitivity of detecting the change of overburden integrity,and the comparative analysis of results with borehole method should be conducted to obtain the best interpre- tation of geophysical results. Then it can be applied to reduce the detection cost of water-conducting zone height.