Free-interface　dual-compatibility　modal　synthesis　method(compatibility　of　both　force　and　displacement　on　interfaces)is　introduced　to　large-scale　civil　engineering　structure　to　enhance　computation　efficiency.　The　basic　equations　of　the　method　are　first　set　up,　and　then　the　mode　cut-off　principle　and　the　dividing　principle　are　proposed.　MATLAB　is　used　for　simulation　in　different　frame　structures.　The　simulation　results　demonstrate　the　applicability　of　this　substructure　method　to　civil　engineering　structures　and　the　correctness　of　the　proposed　mode　cut-off　principle.　Studies　are　also　conducted　on　how　to　divide　the　whole　structure　for　better　computation　efficiency　while　maintaining　better　precision.　It　is　observed　that　the　geometry　and　material　properties　should　be　considered,　and　the　synthesis　results　would　be　more　precise　when　the　inflection　points　of　the　mode　shapes　are　taken　into　consideration.　Furthermore,　the　simulation　performed　on　a　large-scale　high-rise　connected　structure　further　proves　the　feasibility　and　efficiency　of　this　modal　synthesis　method　compared　with　the　traditional　global　method.　It　is　also　concluded　from　the　simulation　results　that　the　fewer　number　of　DOFs　in　each　substructure　will　result　in　better　computation　efficiency,　but　too　many　substructures　will　be　time-consuming　due　to　the　tedious　synthesis　procedures.　Moreover,　the　substructures　with　free　interface　will　introduce　errors　and　reduce　the　precision　dramatically,　which　should　be　avoided.　©　2015,　Tianjin　University　and　Springer-Verlag　Berlin　Heidelberg.