Earthquake-induced　failure　of　underground　structures　attributes　to　their　insufficient　horizontal　deformation　capacity,　which　is　mainly　controlled　by　the　horizontal　deformation　capacity　of　central　columns.　Therefore,　the　developed　measures　to　update　the　seismic　performance　of　underground　structures　are　either　to　enhance　the　deformation　capacity　of　central　columns　or　to　decrease　seismic　loads　transmitted　to　columns.　This　paper　pre-sented　an　underground　structure　seismic　mitigation　system　to　reduce　the　earthquake-induced　damage　of　un-derground　structures　by　decreasing　the　horizontal　deformation　imposed　on　structures.　In　this　system,　frictional　deformation　absorbing　braces　are　set　between　underground　structure　and　earth　retaining　structure.　The　axial　deformation　of　the　braces　is　capable　to　decrease　the　earthquake-induced　horizontal　deformation　of　the　overall　structure.　Furthermore,　axial　force　of　the　braces　can　help　the　earth　retaining　structure　to　resist　the　earthquake-induced　soil　pressure.　The　deformation　behaviour　of　the　frictional　deformation　absorbing　brace　under　axial　loads　was　experimental　firstly　studied.　Experimental　results　presented　that　the　brace　had　a　stable　deformation　property　and　could　provide　expected　braced　force.　Then　seismic　response　of　the　underground　structure　seismic　mitigation　system　was　simulated.　Numerical　results　presented　that　the　seismic　behaviour　of　underground　structure　was　sharply　improved　and　the　structure　experienced　very　small　damage　even　under　strong　earthquakes.　Finally,　the　stiffness　and　strength　of　braces　influencing　the　seismic　performance　of　underground　structure　were　discussed　detailedly.　The　reasonable　stiffness　and　strength　of　the　braces　for　the　target　structure　were　suggested.