Abstract: Considering that an earthquake may cause extreme shattering damage to a mountain and result in the instability of a tunnel’s rock-support structure system after excavation, and using a typically unstable section of the Dujiashan tunnel in a seismically active area as an example, the instability mechanism of the rock-support structure system during construction in shattered soft rock was analyzed from the three aspects of the rock’s basic characteristics, structure instability characteristics and inclinations of the fractured structural plane based on field monitoring and numerical analysis.The results show that the soft rocks in meizoseismal areas are characterized by brokenness, looseness, low density and overhanging so that the structural instability after tunnel excavation mainly manifests in the form of shearing collapse on the tunnel vault and haunch; plastic extrusion, shear sliding and combined failure are the main forms of instability; the most unfavorable condition for tunnel construction is when the maximum deformation around the tunnel, structure stress and plastic zone all occur on the fractured structural plane at 45°.