This　study　is　to　investigate　the　demand　mechanism　of　pounding　required　separation　length　of　high-pier　railway　bridges　at　different　site　soil　conditions　and　also　to　provide　the　evaluation　and　the　computing　theory　method　of　pounding　required　separation　distance　respectively　for　existing　bridges　and　in-built　bridges.　Firstly,　the　spatial　seismic　model　considering　multiple　supported　excitations　was　developed　and　the　mathematical　formulation　was　derived　about　the　relation　of　varied　local　site　soil　conditions　and　acceleration　at　the　bottom　of　proposed　bridge　piers;　Secondly,　the　finite　element　model　of　a　high-pier　railway　bridge　was　developed,　and　the　relation　of　the　separation　distance　between　the　adjacent　bridge　decks　and　the　site　soil　condition　at　various　earthquake　levels　was　obtained;　Finally,　a　real　high-pier　railway　bridge　was　employed　to　analyze　the　required　length　of　adjacent　decks　due　to　different　local　site　soil　conditions　and　the　diverse　earthquake　magnitudes.　The　comparison　analysis　of　the　relative　displacement　responses　at　the　uniform　site　soil　condition　and　the　practical　site　soil　distribution　was　performed.　The　study　indicates　that　the　peak　value　demand　of　power　spectral　density　of　required　separation　length　according　to　uniform　soft　soil　distribution　is　the　maximum,　and　4~6　times　as　large　as　that　in　firm　site　condition　distribution.　The　phase　of　power　spectral　density　function　of　the　required　separation　length　varies　largely　and　the　number　of　deck　pounding　increases　at　real　site　soil　condition　compared　to　the　uniform　site　distribution.　At　different　earthquake　levels,　the　demand　mean　values　of　pounding　required　separation　distance　in　real　site　soil　condition　distribution　are　the　greatest,　and　are　76%,　71%,　and　42%　respectively　greater　compared　to　firm,　medium,　and　soft　site　of　uniform　distributions.　©　2017,　Engineering　Mechanics　Press.　All　right　reserved.