Abstract: Collective effect of long-term structure deformation,unloading and earthquake can result in large-scale deformation and looseness of rock mass under the dynamic condition of regional stability.Large number of weak structural planes developed in relaxed rock mass.The rock mass is completely cracked,very relaxed,highly permeable.Much open-joint and ground fissure exist in the rock mass.The information of the rock breakage is very distinct under the paleo-earthquake.Deformation-relaxed types of relaxed rock mass may be divided simply into five models:i.e.unloading deform-relaxation,toppling deform-relaxation,bedding-slip relaxation,deform-relaxation controlled by faults and relaxation controlled by cranny /joint.The vibration test of dynamic models of rock mass indicates that the earthquake power is the main factor of relaxed effect of the rock mass.The result of numerical simulation shows that large-scale Instability and failure would not occur in the relaxed rock mass under self-weight stress,but it would occur under the earthquake power.