Abstract: The mechanism of water-inrush from coal floor in longwall face is simulated by the similar material tests.The results show that the O-X type failure will occur in the floor without fault but under the water-pressure of the confined aquifer,and the passage of water-inrush will be most easily formed at the point of intersection of "O" and "X";The reason for water-inrush through fault is that the difference of upward relative displacements happens at the key strata of two sides of fault under the confined water pressure,and the more easily the most difference of displacement occurs,the more easily the accident of water-inrush happens,and the fill in the fault zone is eroded by the confined water while the auident of water-inrush occurs.