In　this　paper,　the　multi-tower　suspension　bridge　as　taken　as　research　object.　Based　on　energy　principles,　the　Rayleigh　method　is　used　to　derive　simplified　formulas　of　primary　vertical　frequencies,　whose　modes　are　anti-symmetric　and　symmetrical　vertical　bending　vibration.　During　derivation　processing,　some　secondary　effects　of　suspension　bridge　frequency　factors　are　ignored.　The　accuracy　of　proposed　formulas　in　this　paper　is　verified　by　comparisons　between　the　formulas　and　the　finite　element　approach　for　a　project.　These　formulas　proposed　in　this　paper　can　be　used　to　calculate　primary　vertical　frequencies　for　this　kind　of　bridge.