For　non-classically　damped　linear　systems,　when　the　complex　mode　superposition　method　based　on　seismic　response　spectrum,　i.e.　the　complex　complete　quadratic　combination　(CCQC)　formula,　is　used　for　seismic　response　analysis,　the　amount　of　calculation　is　fairly　large.　Hence,　the　applicability　of　the　CSRSS　(complex　square　root　of　the　sum　of　squares)　method　is　provided　for　studying　the　variations　of　the　modal　displacement,　velocity,　displacement-velocity　correlation　coefficients　along　with　the　ratios　of　different　frequencies　and　damping.　Based　on　these　analyses,　a　simplified　method,　which　only　considers　the　contributions　from　relatively　adjacent　modes　in　CCQC　method　and　is　named　as　simplified　CCQC　or　complex　partial　quadratic　combination　(CPQC)　method,　is　proposed　for　reducing　computational　time.　A　numerical　example　is　presented　to　analyze　the　applicability,　computational　accuracy　and　efficiency.　The　proposed　simplified　method　is　useful　for　classically　damped　linear　systems　as　well,　and　may　also　be　named　as　a　partial　quadratic　combination　(PQC)　method.