Wastewater　treatment　plants　(WWTPs)　in　Beijing　are　currently　undergoing　reconstruction　and　expansion　as　part　of　the　upgrading　of　WWTPs.　The　analysis　of　influent　quality　is　important　for　WWTP　design　as　a　basic　condition　for　evaluating　the　treatment　process.　The　influent　quality　of　two　existing　WWTPs　in　Beijing　was　investigated　using　probability　theory　and　mathematical　statistics.　The　measured　data　values　matched　well　with　the　standard　normal　distribution　curve,　but　had　a　better　consistency　with　the　standard　log-normal　distribution.　Therefore,　the　reference　data　for　influent　quality　is　presented　separately　according　to　a　different　probability　guarantee.　The　results　clearly　show　that　the　actual　influent　quality　is　generally　higher　than　the　current　design　value.　©　2011　IEEE.