Towards　complex　and　viscous　damping　model　of　the　SDOF　system,　this　article　researches　the　differences,similarities　of　the　above　two　system　responses.　Through　calculating　the　two　kinds　of　system　dynamic　equations　based　on　harmonic　excitation　clearly,　the　result　is　that:　When　the　excitation＇s　period　is　less　than　the　natural　period　of　structures,　the　displacement　response　of　the　complex　damping　system　is　stronger　than　the　latter.　Oppositely　,the　speed　response,　the　acceleration　response　are　both　weaker　than　the　latter.　On　the　contrary,　the　result　is　opposite.　Based　on　those　work,　the　acceleration　spectra　of　some　representative　earthquake　records　are　provided　of　the　two　kinds　of　systems,　and　also　the　velocity　and　displacement　spectra.　In　comparison　with　the　last　two　spectra　in　the　condition　of　harmonic　excitation,　the　results　show　the　similar　phenomenon.　But　the　acceleration　spectra　of　the　viscous　damping　model　are　higher　than　that　of　the　complex　damping　model　at　any　time.　And　with　the　growing　damping　ratio,　the　difference　is　more　remarkable.　Under　certain　earthquake　records,　the　structure　displacement　response　values　of　certain　natural　period　may　differ　by　15%.The　velocity　spectra　differ　by　14%.The　acceleration　spectra　differ　by　8%.　In　the　end,　the　article　gives　us　a　suggestion　that　how　to　choose　damping　model　to　make　the　structure　more　Safe　and　reliable.　And　also,　it　provides　some　help　for　the　further　study　of　complex　damping　theory,　references　for　engineering　application.　©　2011　IEEE.