The　epoxide　methoxycarbonylation　is　essential　for　the　synthesis　of　advanced　aldehydes　and　polyols,　but　the　reaction　mechanism　has　not　been　well　resolved　yet.　In　this　work,　we　have　established　more　detailed　mechanism　pathways　for　ethylene　oxide　methoxycarbonylation　over　Co-based　complexes,　the　reaction　pathway　of　the　main　by-product　acetaldehyde,　and　examined　the　effect　of　nitrogen-containing　heterocycles　on　the　pathways　using　DFT　calculation　and　experimental　study.　The　conversion　of　ethylene　oxide　and　product　selectivity　is　sensitive　to　the　structure　of　nitrogen-containing　heterocycles.　The　addition　of　pyrazole　as　a　ligand　generated　the　highest　conversion　(98.3%)　and　dramatically　inhibited　by-product　(e.g.,　acetaldehyde,　2-methoxyethanol)　formation　with　the　highest　selectivity　of　methyl　3-hydroxypropionate　(84.3%).　The　calculations　reveal　that　the　epoxide　ring　opening　is　the　rate-determining　step　and　the　homonuclear　ion　pair　mechanism　is　the　most　energetically　preferred　pathway.　The　ligand　with　promoting　effect　on　the　methyl　3-hydroxypropionate　formation　rate　can　lower　the　energy　barrier　of　the　ring-opening　step　with　a　proportional　correlation　established.