Damping　ratio　is　a　very　important　parameter　for　building　structures　in　the　analysis　of　dynamic　response.　Story-adding　RC　structure　of　light-weight　steel　is　composed　by　two　different　material　stories.　If　all　the　modal　damping　ratios　are　defined　as　the　same,　as　to　the　dynamic　analysis　for　that　structure,　the　results　of　analysis　will　be　inappropriate.　This　paper　made　some　response　spectrum　analysis　on　a　six-story　story-adding　RC　structure　of　light-weight　steel　by　finite　element　analysis　software　Midas　Gen　through　defining　the　Rayleigh　damping.　In　the　meanwhile,　compared　the　results　based　on　different　Rayleigh　damping　parameters　with　the　results　by　using　the　method　of　strain　energy　proportional,　and　got　the　rule　of　the　definition　of　Rayleigh　damping.