Abstract: Through analyzing the expanding effect of rock during destruction process, deformation and instability problem of fault system under mining is studied.As the excavation constantly close to the fault zone, the stress around fault zone is concentrated, which is a loading process for the system composted by fault zone and surrounding medium. When it reaches the critical condition, system failure occurs. The fault reactivation essence is the change of physical and mechanical properties for the system during the loading, which is resulted by the excavation, when the stress is accumulated (caused by the stress concentration) to a critical content.