Abstract: Earthquakes can be extremely damaging to tunnelling through active fault zones.By defining the damage evolution state of concrete model and refining the grading of concrete tensile damage degree by combining the crack width and tensile damage evolution parameter,and then taking the under-construction Taiyuan City Urban Rail Transit Line 1 Phase I project interval tunnel crossing Jiaocheng fault zone as the engineering basis.A three-dimensional refined model simulating the fault displacement was established by using ABAQUS software,and the effects of three key parameters,namely,protection length,joint location and thickness,on the mechanical response and fracture resistance of the tunnel structure with variable cross-section mitigation measures are analysed.The results show that:the use of variable cross-section measures is conducive to reducing the overall damage of the cross-fault underground tunnel structure;the use of tensile damage state classification to assess the damage of the tunnel structure can determine the location and extent of the damage;the tunnel structure is prone to tensile damage when the fault is moving and the damage is widely distributed;the appropriate protection length is necessary to maximise the mitigation effect of the variable cross-section measures;The overall damage to the tunnel structure and the extent of damage can be reduced by selecting the appropriate lining position and thickness;the variable section lining structure effectively controls the extent of lining damage caused by the fault,avoids the occurrence of the tunnel collapse phenomenon,and achieves the purpose of improving the seismic resilience of the underground structure across the fault.