In　this　paper,　frequencies　of　low　order　vertical　modals　for　three-tower　self-anchored　suspension　bridge　(TSSB)　are　studied.　The　first　three-tower　self-anchored　suspension　bridge　is　taken　as　prototype　in　the　paper.　The　finite　element　model　of　the　bridge　is　established.　And　dynamic　characteristics　are　analyzed.　Based　on　Rayleigh　method,　frequency　formulas　of　1st　asymmetric　vertical　vibration　(AVV)　and　symmetric　vertical　vibration　(SVV)　are　deduced　considering　contribution　of　towers　stiffness.　The　accuracy　of　deduced　formulas　was　validated　by　results　of　numerical　analysis　and　modal　test.　Stiffness　characteristic　difference　between　TSSB　and　multi-tower　earth-anchored　suspension　bridge　(MESB)　is　researched　based　on　frequency　formulas.　The　significant　difference　is　gravity　stiffness　component　in　frequency　formulas.　At　last,　influence　law　of　tower　stiffness　on　vertical　frequencies　was　studied　based　on　validated　formulas.　The　results　indicated　that:　1)　middle　tower　stiffness　and　side　tower　stiffness　play　important　role　in　frequency　of　1st　AVV　and　SVV　respectively.　Frequencies　are　enlarging　as　tower　stiffness　increasing;　2)　when　tower　stiffness　is　low,　deduced　frequency　formulas　are　the　same　as　formulas　neglecting　influence　of　tower　stiffness　in　reference　paper;　3)　based　on　deduced　frequency　formulas,　expressions　of　critical　tower　stiffness　for　simplified　formulas　are　proposed.