Floating　system　of　cable-stayed　bridges　under　seismic　action　will　produce　larger　longitudinal　bending　moment　and　displacement,　in　order　to　ensure　the　safety　of　the　structure,　damping　measures　should　be　taken　in　longitudinal　direction.　Taking　a　mainspan　for　416　m　of　the　cable-stayed　bridge　as　an　example,　four　different　damper　layout　schemes　were　analyzed　by　nonlinear　dynamic　time　history　method.　The　results　show　that　installing　viscous　dampers　at　the　abutment　location　to　function　better　comparing　with　other　three　cases.　For　getting　the　best　damping　effect,　the　optimal　parameters　of　damper　are　analyzed.　The　investigation　indicates　that　adding　viscous　dampers　can　significantly　reduce　the　girder　movements,　and　the　reduction　depends　on　damper　parameters.　Finally,　the　optimal　damping　parameters　with　zeta=0.3　and　C=10000　can　be　chosen　for　this　cable-stayed　bridge.