Response　spectrum　method　(RSM)　has　been　applied　in　seismic　analysis　of　underground　structures.　However,　the　RSM　is　computationally　low　efficient　for　processing　large　soil　domains.　In　this　paper,　the　dynamic　characteristics　of　underground　structures　are　first　investigated　through　three　rectangular　subway　station　structures.　The　results　show　that　only　a　small　number　of　effective　frequencies　and　modes　of　soil-structure　interaction　(SSI)　system　significantly　affect　the　seismic　response　of　underground　structure.　These　effective　frequencies　and　modes　are　related　to　the　natural　vibration　characteristics　of　site.　An　effective　method　is　then　developed　to　obtain　the　effective　natural　frequencies　and　modes　of　the　SSI　system.　Subsequently,　the　effective　modal　superposition　response　spectrum　method　(ERSM)　for　seismic　analysis　of　underground　structure　is　developed　by　only　combining　the　effective　natural　frequencies　and　modes　of　the　SSI　system　to　improve　efficiency.　Finally,　the　accuracy　of　ERSM　is　investigated　through　seismic　response　analysis　of　subway　station　structures.　Numerical　examples　show　that　the　ERSM　can　be　used　for　seismic　analysis　of　underground　structures　considering　3D　SSI.