Bitumen　imparts　most　of　its　properties　to　the　asphalt　mixture.　The　chemical　composition　and　molecular　structure　are　the　key　factors　affecting　the　ageing　of　bitumen.　The　objective　of　this　study　is　to　construct　the　average　molecular　structure　of　bitumen　and　study　the　effect　of　ageing　on　its　micro　characteristics.　To　achieve　this　goal,　the　chemical　characteristics　of　bitumen　was　analyzed　by　Fourier　Transform　Infrared　Spectroscopy　(FTIR),　Nuclear　Magnetic　Resonance　(NMR),　Gel　Permeation　Chromatography　(GPC)　and　Elemental　Analysis.　Then　the　average　molecular　structure　parameters　were　calculated　by　the　improved　B-L　method.　Results　show　that　bitumen　is　a　complex　compound　composed　of　aromatic　rings,　naphthenic　and　alkyl　chains.　After　ageing,　the　molecular　mass　and　polydispersity　increased　obviously.　The　oxygen　reacted　with　bitumen　to　form　polar　functional　groups.　The　aromatization　and　ring　scission　may　occur　during　ageing.　There　were　more　aromatic　rings,　less　naphthenic　rings　and　the　ring　structure　became　more　complicated.　©　2021　Taylor　&　Francis　Group,　London.