Safety of mine production was seriously threatened by floor water inrush, the main reason of water inrush was damage of bottom water resisting layer caused by coal mining and detecting depth of destroyed floor was an effective technical mean to prevent water inrush. Taking Hejiabao mine as an example, high precision ARAMIS M/E microseismic monitoring technique was applied in floor destruction of fully mechanized top coal caving face with large mining height in extra thick seam. Firstly, 6 detector probes were arranged with diamond shape in floor bolt of roadway on both sides of working face. Then, optimum wave velocity parameters were selected by least square method and microseismic energy was calculated by vibration duration method. Finally, the failure range of coal seam floor rock mass was determined by analyzing energy, frequency and intensive degree of microseismic events. In order to discuss accuracy of depth of destroyed floor with micro seismic monitoring, analytic method in plastic mechanics was adopted to carry out theoretical calculations. The results showed that microseismic monitoring technique was close to theoretical calculation, which verified reliability of using microseismic monitoring in detecting failure depth of floor.