Far-field　long-period　ground　motions　were　featured　by　long　duration　and　rich　low　frequency　components.　Particularly,　multiple　cycle　pulses　similar　to　harmonic　vibration　were　generated　at　the　late　stage　of　some　seismic　waves.　These　may　cause　adverse　influence　on　the　seismic　performance　of　the　long-period　structures　including　the　isolation　building,　which　needs　to　be　investigated　deeply.　To　this　end,　the　characteristics　of　far-field　long-period　ground　motions　were　discussed.　Then,　six　far-field　long-period　earthquake　records　and　three　common　earthquake　records　were　selected　as　the　input　of　ground　motions,　and　the　seismic　responses　of　a　RC　mid-story　isolated　frame　structures　were　obtained　by　nonlinear　analysis.　Also,　the　impact　of　the　long-period　components　of　seismic　waves,　especially　harmonic-like　components　on　shock　absorption　performance,　was　investigated.　Further,　a　combined　isolation　scheme　by　the　addition　of　viscous　dampers　in　the　isolation　layer　was　proposed,　and　its　effects　on　limiting　deformation　of　the　isolation　layer　and　reducing　nonlinear　responses　were　explored.　The　results　show　that　the　effectiveness　of　the　mid-story　isolation　subjected　to　far-field　long-period　ground　motions　on　reducing　nonlinear　seismic　responses　is　not　as　good　as　those　subjected　to　common　ground　motions.　Particularly,　the　nonlinear　responses　of　the　mid-story　isolated　structure　are　seldom　reduced　or　significantly　enlarged　under　the　far-field　harmonic-like　strong　ground　motions　compared　with　those　of　the　seismic　structure,　and　the　maximum　displacement　of　isolation　bearings　is　far　beyond　the　allowable　displacement.　It　is　also　indicated　that　the　combined　isolation　can　effectively　control　nonlinear　responses　of　the　mid-story　isolated　structure　under　far-field　long-period　ground　motions,　especially　harmonic-like　ground　motions.　In　particular,　the　maximum　displacement　of　the　isolation　bearings　can　be　significantly　decreased,　preventing　the　destruction　of　the　isolation　bearing.　©　2018,　Editorial　Office　of　Journal　of　Vibration　and　Shock.　All　right　reserved.