The　safety　and　reliability　of　the　double-deck　variable-depth　composite　truss　bridge　(DDVD-CTB)　during　earthquakes　can　be　ensured　by　a　hybrid　friction-based　seismic　isolation　system　(HFBSIS).　However,　the　overturning　effects　on　seismic　performance　of　HFBSIS　is　a　significant　concern,　as　it　can　induce　vertical　actions　of　isolation　bearings.　In　this　study,　a　single-degree-of-freedom　(SDOF)　system　was　first　employed　to　describe　the　isolation　system　with　the　consideration　of　overturning　effects.　Following　the　nondimensionalization　of　motion　equation,　it　indicated　that　the　ratio　of　total　axial　load　variations　of　spherical　bearings　to　deadweight　of　the　girder　(γ)　was　a　good　indicator　for　the　overturning　effects　on　seismic　performance.　Then,　the　finite　element　(FE)　model　of　SDOF　system　was　developed　and　verified　by　the　quasi-static　cyclic　test　results.　Finally,　parametric　sensitivity　analyses　were　carried　out　to　evaluate　the　degree　of　overturning　effects　on　the　seismic　performance　of　the　isolation　system　under　different　design　parameters.　Therefore,　the　maximum　value　γ　of　was　less　than　5　×　10−7　degree　in　the　engineering　practice.　The　nondimensionalization　of　motion　equation　considering　overturning　effects　was　approximately　the　same　as　that　neglecting　overturning　effects,　suggesting　that　overturning　effects　could　be　neglected　when　designing　the　HFBSIS　in　DDVD-CTB.　©　2022　Elsevier　Ltd