Abstract: In view of the potential instability of surrounding rock in large-scale underground mining in Sijiaying， UDEC software is used to carry out numerical calculation to analyze the variation of ground pressure in different mining stages. Stress nephogram reveals the evolution of arch bearing structure in roof rock mass of stope， which is beneficial to maintaining the stability of surrounding rock. The arch rise f and arch span L in this structure are approximately equal. At the same time， there are three high pressure stress zones in the upper and lower boundaries of orebody at the arch foot and the rock strata of the arch roof weathering zone. The stress concentration in the upper and lower wall suggests that the ore room in this area should be filled quickly. In addition， based on the results of numerical analysis， the arch truss structure engineering system is established. The mechanical model of the system is analyzed by the force method， and the width ratio of one-step room to two-step room is obtained. The width of the room with thick and large orebody is designed to be 20 m， with the safety factor of 1.96. The safety and continuity of large-scale underground mining can be effectively guaranteed by early warning of numerical calculation， and optimization design based on theoretical analysis.

Key words: Large and thick orebody, Open stoping with subsequent filling method, Numerical simulation, Truss Structure, Stability of surrounding rock