Abstract: To study the mechanism of fault damage activation, the mechanical models for fracture damage development and rock mass slipping under water pressure were established based on fracture mechanics.From the two mechanism models, the main influencing factors including the ratio of horizontal stress to vertical stress λ, whether there was water or not in fault zone, elastic modulus E, Poisson's ratio μ, fault angle θ₁, internal friction angle φ and cohesion c and so on were obtained.At the same time, the results were verified by numerical simulation using FLAC^3D built-in FISH language according to the criterion of fault activation degree based on the value of step subsidence at fault outcrop.The results show that the possibility of fault activation would increase with λ when it is smaller than 1, while the possibility would decrease when λ increases and is larger than 1 for the reason that the fault zone fracture is easier to expand when λ is smaller than 1 and increasing, while it's adverse when λ is larger than 1.When there is water existing in fault zone, water would promote possibility of fault activation for water pressure wouldn't only expand the fracture, but also lubricate the rock mass sliding.With the increase of E, the original fracture of the rock mass in the fault zone is not easy to expand, and the possibility of fault activation is reduced, while it's reverse according to μ.The step subsidence value at the fault outcrop depends on normal subsidence value at fault outcrop without fault existing and rock mass slipping value influenced by mining, the rock mass slipping value would decrease at fault outcrop when θ₁ increases, but the normal subsidence would increase sharply.As the result, the step subsidence value at fault outcrop increases as well as the possibility of fault activation.At the same time, when internal friction angle φ and cohesion c increase, the fault zone fracture is harder to expand and the possibility of fault activation decreases.