Response　spectrum　method　(RSM)　has　been　applied　in　seismic　analysis　of　underground　structures　considering　linear　soil-structure　interaction　(SSI).　However,　the　RSM　has　two　main　shortcomings:　it　does　not　account　for　soil　nonlinearity　and　it　is　computationally　inefficient　for　processing　large　soil　domains.　This　paper　addresses　these　shortcomings　by　extending　the　RSM　to　consider　soil　nonlinearity　and　by　simplifying　the　modal　analysis　of　SSI　systems.　The　soil　nonlinearity　is　considered　by　transforming　the　one-dimensional　(1D)　frequency-domain　equivalent　linearization　model　into　a　modal　damping　linear　elastic　model,　in　which　two　explicit　formulations　of　the　modal　damping　ratio　are　presented.　The　resulting　degraded　shear　modulus　and　modal　damping　ratio　are　applied　to　the　SSI　model　to　consider　soil　nonlinearity.　In　addition,　it　was　found　that　a　small　number　of　natural　frequencies　and　mode　shapes　of　the　SSI　system　are　already　sufficient　to　evaluate　vibration　characteristics　in　a　typical　2D　SSI　analysis.　Accordingly,　the　first　three　natural　vibration　modes　of　the　1D　soil　site　are　used　as　the　loading　condition　to　determine　the　effective　characteristics　of　the　SSI　system　by　iterative　static　analysis　of　the　SSI　system.　The　effectiveness　of　the　proposed　procedure　in　considering　soil　nonlinearity　and　avoiding　modal　analysis　of　the　SSI　system　is　verified　by　comparisons　with　numerical　examples.　Finally,　the　extended　RSM　is　applied　to　the　seismic　design　of　three　prototype　underground　structures.