Abstract: Theoretical analysis is taken on the water sediment two phase flows pattern of fractured rock based on lattice Boltzmann method and a model is built. Also,the basic conservation equations are derived and the control equations are established. The water and sand phase interface is analyzed by using the Euler point and Lagrange point based on the immersed boundary method. Numerical simulation is taken on sand inrush,and the flow patterns of sand moving in fracture is analyzed,the influence of particle size and fracture width on the velocity of sand inrush is studied. With the results in single fracture,a mechanical model of sand mining roof covered with thick loose is established to analyze wa- ter and sand inrush,the authors find that at the initial stage,the sand particles flowing in cracks can restrain its devel- opment,with the water pressure continually applied,a large number of water and sand mixture begin to flow in,which makes the pore pressure increase,causing the cracks rapidly develop,even causing the roof fall,aggravating the water and sand inrush.