To　study　the　lateral　elastic-plastic　seismic　performance　and　failure　mechanism　of　diamond-shaped　pylon　in　long　span　cable-stayed　bridges　under　extreme　seismic　excitation,　a　typical　long　span　cable-stayed　bridge　was　taken　as　the　prototype　and　its　finite　element　model　was　established　using　SAP2000.　According　to　the　principle　of　self-weight　effect,　lateral　stiffness　and　lateral　mode　equivalence,　a　simplified　method　of　diamond-shaped　pylon　is　proposed.　The　entire　bridge　model　is　simplified　into　an　equivalent　single-tower　model.　Two　fiber　finite　element　models　were　established　in　OpenSees.　Comparative　analyses　of　static　characteristics　and　related　results　were　conducted　to　assess　the　applicability　of　single-towers　with　different　material　property.　The　reliability　of　the　models　was　verified　by　comparing　quasi-static　test　results.　A　systematic　investigation　of　the　lateral　seismic　performance　of　diamond-shaped　pylon　was　carried　out　at　three　hierarchical　levels:　the　structure,　cross-section,　and　material.　The　analysis　identified　the　yielding　sequence　and　damage　mechanisms.　An　in-depth　analysis　of　material　damage　in　diamond-shaped　pylon　offers　essential　data　to　support　the　investigation　of　their　failure　mechanisms　under　severe　seismic　excitations.　The　analysis　results　reveal　that　the　lower　area　of　the　middle　crossbeam　is　the　most　vulnerable　region　in　the　diamond-shaped　pylon.　The　failure　mechanisms　consist　of　tensile　yielding　of　the　steel　reinforcement,　subsequent　cracking　compressive　strain　in　the　concrete,　and　final　concrete　crushing　failure.　©　2025　Institution　of　Structural　Engineers