Before　the　hardening　of　concrete　core　in　construction　of　a　Concrete　Filled　Steel　Tubular　(CFST)　arch,　the　steel　tubular　rib　will　bear　itself　weight　and　the　fresh　concrete.　Thus　initial　stresses　will　be　built　up　in　it.　Total　of　4　single　circular　CFST　arch　models　with　various　initial　stress　degrees　were　tested;　and　parametric　analyses　by　the　finite　element　method　of　the　influence　of　initial　stress　to　the　ultimate　load-carrying　capacities　of　such　a　structure　were　carried　out.　The　results　indicate　that　the　initial　stress　will　not　influence　the　failure　model　but　will　make　the　ultimate　load　decrease.　The　influence　of　initial　degree　to　the　ultimate　load　of　CFST　arch　has　a　strong　relationship　to　arch　slenderness　and　its　rise-to-span　ratio,　but　weak　to　load　case.　The　ultimate　load　carrying　capacity　of　a　CFST　single　tubular　arch　with　initial　stress　can　be　obtained　by　multiplying　a　reduction　factor　Kp　with　the　load　carrying　capacity　Pu0　of　CFST　arch　without　initial　stress.　The　factor　Kp　can　be　calculated　by　multiplying　the　two　reduction　factors,　one　related　to　slenderness　and　the　other　to　rise-to-span　ratio,　whereas　the　Pu0　can　be　attained　by　the　revised　equivalent　beam-column　method.