Abstract: To determine the reasonable thickness and position of contact lane closed wall at fully mechanized caving face, we identified mechanics parameters and numerical simulation constitutive model, boundary conditions of fly ash material and rock mass. We use ansys software to establish fully mechanized caving face contact lane closed wall and set a numerical model parameters and stress-related conditions by FLAC^3D software to analyze different conditions closed wall stress, deformation and destruction, and study the impact of different positions and thickness on closed wall stress field, deformation field and plastic failure field. Simulation results show that when contacted lane length is 20 m, the reasonable distance of closed wall is from the location of the roadway at the intersection of 15 m; reasonable airtight wall width is 4.9 m.