Sulfur　and　polyphosphoric　acid　(PPA)　were　used　to　improve　the　thermal　stability　and　high-temperature　property　of　styrene-butadiene　rubber　(SBR)-modified　asphalt　before　and　after　short-term　and　long-term　aging.　The　physical,　rheological　properties,　and　thermal　behavior　of　asphalt　binders　were　studied.　The　study　showed　that　the　addition　of　PPA　improved　the　thermal　stability　and　high-temperature　property　of　SBR-modified　asphalt　evidently,　and　the　improved　property　caused　by　the　gelation　effect　of　PPA　was　not　influenced　by　aging　further.　Though　sulfur　also　improved　thermal　stability　and　high-temperature　property　of　SBR-modified　asphalt　before　aging,　the　improved　properties　declined　evidently　with　further　aging,　due　to　the　susceptibility　of　SBR/sulfur-modified　asphalt　to　aging.　It　is　reasonable　to　assume　that　the　SBR-modified　asphalt　could　be　improved　further　only　by　the　addition　of　PPA.　In　the　study　of　polymer-modified　asphalts,　thermodynamic　analysis　can　reflect　the　structural　characteristics　of　asphalt　binders　before　and　after　aging　efficiently　and　confirms　the　conclusions　of　physical　and　rheological　testings　to　some　extent.