In　order　to　analyze　rationality　of　post　speed　limit,　authors　select　the　first　grade　highway　Xinjiang　Saibo　road　as　a　study　object.　Three　study　sites　were　posted　in　the　vicinity　of　speed　limit　sign.　By　exploring　relationship　between　actual　speed　and　post　speed　limit　inside　and　outside　speed　zone,　the　rationality　of　post　limit　sign　can　be　evaluated.　The　analysis　results　lays　foundation　for　post　speed　determination　and　also　for　informs　decision　makers　to　be　aware　where　speeding　is　most　prevalent.　It　is　concluded　that　on　the　premise　of　traffic　safety,　the　PSL　should　optimize　the　use　of　limited　road　resources.